Bug 1300655 - part 2 - WorkerProxyToMainThreadRunnable::PostDispatchOnMainThread::ReleaseRunnable should implement Cancel() correctly, r=me

dom/workers/WorkerRunnable.cpp

@@ -730,10 +730,14 @@ WorkerProxyToMainThreadRunnable::PostDispatchOnMainThread()
       MOZ_ASSERT(aWorkerPrivate);
       aWorkerPrivate->AssertIsOnWorkerThread();
 
-      mRunnable->RunBackOnWorkerThread();
+      if (mRunnable) {
+        mRunnable->RunBackOnWorkerThread();
+
+        // Let's release the worker thread.
+        mRunnable->ReleaseWorker();
+        mRunnable = nullptr;
+      }
 
-      // Let's release the worker thread.
-      mRunnable->ReleaseWorker();
       return true;
     }