Use text for webSocketUrl. (#606)

See https://w3c.github.io/webdriver/#ref-for-dfn-remote-end-steps-6

In step 4 we process capabilities and (assuming there's a match and we're starting a websocket connection), end up with an object that contains {webSocketUrl: true}.

In step 9 we call into https://w3c.github.io/webdriver-bidi/#establishing which updates the capabilities object so that it contains the actual URL as the value of the webSocketUrl property.

index.bs

@@ -1594,7 +1594,7 @@ This is a [=static command=].
           platformName: text,
           setWindowRect: bool,
           ? proxy: session.ProxyConfiguration,
-          ? webSocketUrl: bool,
+          ? webSocketUrl: text / true,
           Extensible
         }
       }