Bug 1318506 - Label AsyncEventDispatcher runnables with DocGroup (r=ehsan)

I still don't completely understand why we sometimes need to use the global
and sometimes the node. As far as I understand it:
- Not all event targets are nodes, so the node code can't always be used.
- The nsINode::GetOwnerGlobal implementation uses GetScriptHandlingObject,
  which returns null in some cases where GetScopeObject doesn't. Here
  is one:
    http://searchfox.org/mozilla-central/rev/62db1c9021cfbde9fa5e6e9601de16c21f4c7ce4/dom/base/nsDocument.cpp#4627

MozReview-Commit-ID: DdLWeQJIWZx

dom/events/AsyncEventDispatcher.cpp

@@ -66,6 +66,18 @@ nsresult
 AsyncEventDispatcher::PostDOMEvent()
 {
   RefPtr<AsyncEventDispatcher> ensureDeletionWhenFailing = this;
+  if (NS_IsMainThread()) {
+    if (nsCOMPtr<nsIGlobalObject> global = mTarget->GetOwnerGlobal()) {
+      return global->Dispatch("AsyncEvent", TaskCategory::Other, ensureDeletionWhenFailing.forget());
+    }
+
+    // Sometimes GetOwnerGlobal returns null because it uses
+    // GetScriptHandlingObject rather than GetScopeObject.
+    if (nsCOMPtr<nsINode> node = do_QueryInterface(mTarget)) {
+      nsCOMPtr<nsIDocument> doc = node->OwnerDoc();
+      return doc->Dispatch("AsyncEvent", TaskCategory::Other, ensureDeletionWhenFailing.forget());
+    }
+  }
   return NS_DispatchToCurrentThread(this);
 }