Add browsingContext.historyUpdated event (#740)

Adds an event that notifies the client whenever a history is updated.

index.bs

@@ -2711,6 +2711,7 @@ BrowsingContextEvent = (
   browsingContext.DomContentLoaded //
   browsingContext.DownloadWillBegin //
   browsingContext.FragmentNavigated //
+  browsingContext.HistoryUpdated //
   browsingContext.Load //
   browsingContext.NavigationAborted //
   browsingContext.NavigationFailed //
@@ -4753,6 +4754,48 @@ navigated</dfn> steps given |navigable| and |navigation status|:
 
 </div>
 
+#### The browsingContext.historyUpdated Event #### {#event-browsingContext-historyUpdated}
+
+<dl>
+   <dt>Event Type</dt>
+   <dd>
+      <pre class="cddl local-cddl">
+        browsingContext.HistoryUpdated = (
+          method: "browsingContext.historyUpdated",
+          params: browsingContext.HistoryUpdatedParameters
+        )
+
+        browsingContext.HistoryUpdatedParameters = {
+          context: browsingContext.BrowsingContext,
+          url: text
+        }
+      </pre>
+   </dd>
+</dl>
+
+<div algorithm>
+The [=remote end event trigger=] is the <dfn export>WebDriver BiDi history updated</dfn> steps given |navigable|:
+
+1. Let |url| be the result of running the [=URL serializer=],
+   given |navigable|'s [=active browsing context=]'s [=active document=]'s <a spec=dom>URL</a>.
+
+1. Let |params| be a [=/map=] matching the <code>browsingContext.HistoryUpdatedParameters</code> production,
+   with the <code>url</code> field set to |url| and
+   the <code>context</code> field set to |navigable|'s [=navigable id=].
+
+1. Let |body| be a [=/map=] matching the
+   <code>browsingContext.HistoryUpdated</code> production, with the
+   <code>params</code> field set to |params|.
+
+1. Let |related browsing contexts| be a [=/set=] containing |navigable|'s [=active browsing context=].
+
+1. For each |session| in the [=set of sessions for which an event is enabled=]
+   given "<code>browsingContext.historyUpdated</code>" and |related browsing contexts|:
+
+  1. [=Emit an event=] with |session| and |body|.
+
+</div>
+
 #### The browsingContext.domContentLoaded Event #### {#event-browsingContext-domContentLoaded}
 
 <dl>