Support user prompt handlers for BiDi sessions

The key part of this is that with BiDi HTML calls into the spec when
it's going to show a prompt. So at that point we can see if there's a
handler defined, and if so return the handler to HTML.

All the actual logic to handle the dialogs ends up in the HTML spec.

From the BiDi point of view, the intended lifecycle is that you always
get the session.userPromptOpened and session.userPromptClosed events,
even if the prompt is automatically handled. But the
session.UserPromptOpened event gets a new `handler` property that
tells you whether the prompt will be automatically handled; if this is
set to "none" the automation has to send a session.handleUserPrompt
command (or wait for the prompt to be dismissed by the user).

index.bs

@@ -198,6 +198,9 @@ spec: HTML; urlPrefix: https://html.spec.whatwg.org/multipage/
     text: window open steps; url: window-object.html#window-open-steps
     text: worker event loop; url: webappapis.html#worker-event-loop-2
     text: worklet global scopes; url:worklets.html#concept-document-worklet-global-scopes
+spec: INFRA; urlPrefix: https://infra.spec.whatwg.org/
+  type: dfn
+    text: convert a JSON-derived JavaScript value to an Infra value; url: convert-a-json-derived-javascript-value-to-an-infra-value
 spec: RESOURCE-TIMING; urlPrefix: https://w3c.github.io/resource-timing/
   type: dfn
     text: convert fetch timestamp; url: dfn-convert-fetch-timestamp
@@ -408,7 +411,7 @@ WebDriver BiDi extends the [=/session=] concept from [[WEBDRIVER|WebDriver]].
 A [=/session=] has a <dfn>BiDi flag</dfn>, which is false unless otherwise
 stated.
 
-A <dfn>BiDi session</dfn> is a [=/session=] which has the [=BiDi flag=]
+A <dfn export>BiDi session</dfn> is a [=/session=] which has the [=BiDi flag=]
 set to true.
 
 <div algorithm>
@@ -1410,13 +1413,36 @@ session.CapabilityRequest = {
     ? socksProxy: text,
     ? socksVersion: 0..255,
   },
+  ? userPromptHandler: session.UserPromptHandler,
   Extensible
 };
 </pre>
 
 The <code>session.CapabilityRequest</code> type represents a specific set of
 requested capabilities.
 
+#### The session.UserPromptHandler Type #### {#type-session-UserPromptHandler}
+
+[=remote end definition=] and [=local end definition=]
+
+<pre class="cddl remote-cddl local-cddl">
+session.UserPromptHandler = session.UserPromptHandlerType /
+                            session.UserPromptHandlerConfiguration;
+
+session.UserPromptHandlerType = "accept" / "dismiss" / "ignore";
+
+session.UserPromptHandlerConfiguration = {
+  ? alert: session.UserPromptHandlerType,
+  ? beforeUnload: session.UserPromptHandlerType,
+  ? confirm: session.UserPromptHandlerType,
+  ? default: session.UserPromptHandlerType,
+  ? prompt: session.UserPromptHandlerType,
+};
+</pre>
+
+The <code>session.UserPromptHandler</code> type represents the configuration of
+the user prompt handler.
+
 #### The session.SubscriptionRequest Type #### {#type-session-SubscriptionRequest}
 
 <pre class="cddl remote-cddl">
@@ -1525,6 +1551,7 @@ This is a [=static command=].
             ? socksVersion: 0..255,
           },
           setWindowRect: bool,
+          ? userPromptHandler: session.UserPromptHandler,
           Extensible
         }
       }
@@ -1543,9 +1570,12 @@ The [=remote end steps=] given |session| and |command parameters| are:
 
 1. Let |flags| be a [=/set=] containing "<code>bidi</code>".
 
-1. Let |capabilities| be the result of [=trying=] to [=process capabilities=]
+1. Let |capabilities json| be the result of [=trying=] to [=process capabilities=]
    with |command parameters| and |flags|.
 
+1. Let |capabilities| be [=convert a JSON-derived JavaScript value to an Infra
+   value=] with |capabilities json|.
+
 1. Let |session| be the result of [=trying=] to [=create a session=] with
    |capabilities| and |flags|.
 
@@ -3598,8 +3628,9 @@ closed</dfn> steps given |window|, |accepted| and optional |user text|
 
         browsingContext.UserPromptOpenedParameters = {
           context: browsingContext.BrowsingContext,
-          type: "alert" / "confirm" / "prompt" / "beforeunload",
+          handler: "accept" / "dismiss" / "ignore",
           message: text,
+          type: "alert" / "confirm" / "prompt" / "beforeunload",
           ? defaultValue: text
         }
       </pre>
@@ -3615,12 +3646,18 @@ opened</dfn> steps given |window|, |type|, |message|, and optional |default valu
 
 1. Let |context id| be the [=browsing context id=] for |context|.
 
+1. Let |handler configuration| be [=get the prompt handler=] with |type|.
+
+1. Let |handler| be |handler configuration|'s [=prompt handler
+   configuration/handler=].
+
 1. Let |params| be a [=/map=] matching the
    <code>browsingContext.UserPromptOpenedParameters</code> production with the
    <code>context</code> field set to |context id|, the <code>type</code> field
-   set to |type|, the <code>message</code> field set to |message|, and the
+   set to |type|, the <code>message</code> field set to |message|, the
    <code>defaultValue</code> field set to |default value| if |default value| is
-   not null or omitted otherwise.
+   not null or omitted otherwise, and the <code>handler</code> field set to
+   |handler|.
 
 1. Let |body| be a [=/map=] matching the
    <code>browsingContext.UserPromptOpened</code> production, with the
@@ -3633,13 +3670,9 @@ opened</dfn> steps given |window|, |type|, |message|, and optional |default valu
 
   1. [=Emit an event=] with |session| and |body|.
 
-1. If |type| is "<code>beforeUnload</code>":
-
-  1. For each |session| in [=active sessions=]:
+1. If |handler| is "<code>ignore</code>", set handler to "<code>none</code>".
 
-    1. If |session|'s [=http flag=] is true, return "<code>accept</code>".
-
-1. Return null.
+1. Return |handler|
 
 </div>