Bug 1820210 - Worker improvements. r=dom-worker-reviewers,edenchuang a=RyanVM

Differential Revision: https://phabricator.services.mozilla.com/D171779

dom/workers/Worker.cpp

@@ -85,6 +85,8 @@ void Worker::PostMessage(JSContext* aCx, JS::Handle<JS::Value> aMessage,
   if (!mWorkerPrivate || mWorkerPrivate->ParentStatusProtected() > Running) {
     return;
   }
+  RefPtr<WorkerPrivate> workerPrivate = mWorkerPrivate;
+  Unused << workerPrivate;
 
   JS::Rooted<JS::Value> transferable(aCx, JS::UndefinedValue());
 
@@ -140,6 +142,10 @@ void Worker::PostMessage(JSContext* aCx, JS::Handle<JS::Value> aMessage,
 
   runnable->Write(aCx, aMessage, transferable, clonePolicy, aRv);
 
+  if (!mWorkerPrivate || mWorkerPrivate->ParentStatusProtected() > Running) {
+    return;
+  }
+
   if (isTimelineRecording) {
     end = MakeUnique<WorkerTimelineMarker>(
         NS_IsMainThread()