Bug 1379243 P6 Override Pre/Post dispatch methods in ExternalRunnableWrapper so it can be used off main thread. r=baku

dom/workers/WorkerPrivate.cpp

@@ -240,6 +240,19 @@ private:
   ~ExternalRunnableWrapper()
   { }
 
+  virtual bool
+  PreDispatch(WorkerPrivate* aWorkerPrivate) override
+  {
+    // Silence bad assertions.
+    return true;
+  }
+
+  virtual void
+  PostDispatch(WorkerPrivate* aWorkerPrivate, bool aDispatchResult) override
+  {
+    // Silence bad assertions.
+  }
+
   virtual bool
   WorkerRun(JSContext* aCx, WorkerPrivate* aWorkerPrivate) override
   {