Add Event's cancelBubble attribute

More delightful garbage from the past.

Tests: web-platform-tests/wpt#4304.

Fixes #211.

Author: annevk

dom.bs

@@ -511,6 +511,7 @@ interface Event {
   readonly attribute unsigned short eventPhase;
 
   void stopPropagation();
+           attribute boolean cancelBubble; // historical alias of .stopPropagation
   void stopImmediatePropagation();
 
   readonly attribute boolean bubbles;
@@ -718,6 +719,12 @@ flags that are all initially unset:
 <p>The <dfn method for=Event><code>stopPropagation()</code></dfn> method, when invoked, must set the
 <a>context object</a>'s <a>stop propagation flag</a>.</p>
 
+<p>The <dfn attribute for=Event><code>cancelBubble</code></dfn> attribute's getter must return true
+if <a>context object</a>'s <a>stop propagation flag</a> is set, and false otherwise.
+
+<p>The {{Event/cancelBubble}} attribute's setter must set <a>context object</a>'s
+<a>stop propagation flag</a> if the given value is true, and do nothing otherwise.
+
 <p>The <dfn method for=Event><code>stopImmediatePropagation()</code></dfn> method, when invoked,
 must set <a>context object</a>'s <a>stop propagation flag</a> and <a>context object</a>'s
 <a>stop immediate propagation flag</a>.</p>