Bug 1840313 fetch and remove the first element of queue in one operation r=emilio

The two-step logic was left over from a previous std::queue implementation.
https://hg.mozilla.org/mozilla-central/rev/4534179de22c013eb97e8226a538096d7a648721#l2.36

Depends on D181966

Differential Revision: https://phabricator.services.mozilla.com/D181967

xpcom/threads/TaskQueue.cpp

@@ -238,8 +238,7 @@ nsresult TaskQueue::Runner::Run() {
       mon.NotifyAll();
       return NS_OK;
     }
-    event = std::move(mQueue->mTasks.FirstElement());
-    mQueue->mTasks.Pop();
+    event = mQueue->mTasks.Pop();
   }
   MOZ_ASSERT(event.event);