Add AbortSignal.abort() static method

Returns an aborted AbortSignal.

Tests: web-platform-tests/wpt#28003.

Fixes #959.

dom.bs

@@ -1780,12 +1780,17 @@ constructor steps are:
 <pre class="idl">
 [Exposed=(Window,Worker)]
 interface AbortSignal : EventTarget {
+  [NewObject] static AbortSignal abort();
+
   readonly attribute boolean aborted;
 
   attribute EventHandler onabort;
 };</pre>
 
 <dl class=domintro>
+ <dt><code>AbortSignal . <a method for="AbortSignal">abort()</a></code>
+ <dd>Returns an {{AbortSignal}} instance whose <a for=AbortSignal>aborted flag</a> is set.
+
  <dt><code><var>signal</var> . <a attribute for=AbortSignal>aborted</a></code>
  <dd>Returns true if this {{AbortSignal}}'s {{AbortController}} has signaled to abort, and false
  otherwise.
@@ -1817,6 +1822,16 @@ requirements to react in a reasonable way to {{AbortController/abort()}}. For ex
 [=AbortSignal/aborted flag=] might need to be propagated to a cross-thread environment, such as a
 service worker.
 
+<p>The static <dfn method for=AbortSignal><code>abort()</code></dfn> method steps are:
+
+<ol>
+ <li><p>Let <var>signal</var> be a new {{AbortSignal}} object.
+
+ <li><p>Set <var>signal</var>'s [=AbortSignal/aborted flag=].
+
+ <li>Return <var>signal</var>.
+</ol>
+
 <p>The <dfn attribute for=AbortSignal>aborted</dfn> getter steps are to return true if <a>this</a>'s
 [=AbortSignal/aborted flag=] is set; otherwise false.
 
@@ -10036,6 +10051,7 @@ Jake Archibald<!-- technically B.J. Archibald -->,
 Jake Verbaten,
 James Graham,
 James Greene,
+James M Snell,
 James Robinson,
 Jeffrey Yasskin,
 Jens Lindström,