Bug 1202667 - make TaskQueue task running slightly more efficient; r=mccr8

We can transfer the reference out of the queue of runnables instead of
taking a new reference right before we drop the old one.

xpcom/threads/TaskQueue.cpp

@@ -155,7 +155,7 @@ TaskQueue::Runner::Run()
       mon.NotifyAll();
       return NS_OK;
     }
-    event = mQueue->mTasks.front();
+    event = mQueue->mTasks.front().forget();
     mQueue->mTasks.pop();
   }
   MOZ_ASSERT(event);