Bug 1527978, bring back the old behavior of worker's postMessage spam limit, r=baku

--HG--
extra : rebase_source : 2a124802dd0999da3f1e08b4882d6660cea08fc7

dom/workers/WorkerPrivate.cpp

@@ -2703,7 +2703,8 @@ void WorkerPrivate::DoRunLoop(JSContext* aCx) {
     size_t queuedEvents = mMainThreadEventTarget->Length() +
                           mMainThreadDebuggeeEventTarget->Length();
     if (queuedEvents > 5000) {
-      mMainThreadEventTarget->AwaitIdle();
+      // Note, postMessage uses mMainThreadDebuggeeEventTarget!
+      mMainThreadDebuggeeEventTarget->AwaitIdle();
     }
   }