Bug 1288033 - Decease the busyCount when Cancel() is called in WorkerRunnable, r=khuey

dom/workers/WorkerRunnable.cpp

@@ -260,6 +260,10 @@ WorkerRunnable::Run()
 
     MOZ_ASSERT(IsCanceled(), "Subclass Cancel() didn't set IsCanceled()!");
 
+    if (mBehavior == WorkerThreadModifyBusyCount) {
+      mWorkerPrivate->ModifyBusyCountFromWorker(WorkerPrivate::eDecreaseBusyCount);
+    }
+
     return NS_OK;
   }