Bug 1259338 P1 Ensure that AllowWindowInteractionHandler is released on worker thread. r=khuey

dom/workers/ServiceWorkerPrivate.cpp

@@ -845,6 +845,15 @@ private:
   bool
   WorkerRun(JSContext* aCx, WorkerPrivate* aWorkerPrivate) override;
 
+  NS_IMETHOD
+  Cancel() override
+  {
+    // Always ensure the handler is released on the worker thread, even if we
+    // are cancelled.
+    mHandler = nullptr;
+    return WorkerRunnable::Cancel();
+  }
+
   RefPtr<AllowWindowInteractionHandler> mHandler;
 };
 
@@ -955,6 +964,7 @@ bool
 ClearWindowAllowedRunnable::WorkerRun(JSContext* aCx, WorkerPrivate* aWorkerPrivate)
 {
   mHandler->ClearWindowAllowed(aWorkerPrivate);
+  mHandler = nullptr;
   return true;
 }