Bug 1434580, ensure proper mouseover handling, r=masayuki

dom/events/EventStateManager.cpp

@@ -4320,9 +4320,10 @@ EventStateManager::NotifyMouseOver(WidgetMouseEvent* aMouseEvent,
   // content associated with our subdocument.
   EnsureDocument(mPresContext);
   if (nsIDocument *parentDoc = mDocument->GetParentDocument()) {
-    if (nsIContent *docContent = parentDoc->FindContentForSubDocument(mDocument)) {
+    if (nsCOMPtr<nsIContent> docContent =
+          parentDoc->FindContentForSubDocument(mDocument)) {
       if (nsIPresShell *parentShell = parentDoc->GetShell()) {
-        EventStateManager* parentESM =
+        RefPtr<EventStateManager> parentESM =
           parentShell->GetPresContext()->EventStateManager();
         parentESM->NotifyMouseOver(aMouseEvent, docContent);
       }