Bug 1332322 part 1. Add an nsWrapperCache function to mark the wrapper as 'live' for GC purposes. r=mccr8

dom/xhr/XMLHttpRequestMainThread.cpp

@@ -321,8 +321,7 @@ NS_IMPL_CYCLE_COLLECTION_CAN_SKIP_BEGIN(XMLHttpRequestMainThread)
       tmp->mListenerManager->MarkForCC();
     }
     if (!isBlack && tmp->PreservingWrapper()) {
-      // This marks the wrapper black.
-      tmp->GetWrapper();
+      tmp->MarkWrapperLive();
     }
     return true;
   }