Introduce WorkerThread::willShutdown()

Source/core/workers/WorkerThread.h

Index: Source/core/workers/WorkerThread.h
diff --git a/Source/core/workers/WorkerThread.h b/Source/core/workers/WorkerThread.h
index 1685abcf7afdde37cbd03a3229a6ca17e5e57cee..3289acff1ecc3b505bf911559261991b8e011fc2 100644
--- a/Source/core/workers/WorkerThread.h
+++ b/Source/core/workers/WorkerThread.h
@@ -133,6 +133,12 @@ protected:
     virtual void shutdownBackingThread();
 
     virtual v8::Isolate* initializeIsolate();
+    // willShutdown() is called just before dispoing WorkerGlobalScope and its
+    // WorkerScriptController. If you want to run any clean-up task that needs to
+    // interact with JavaScript, you should run the task in willShutdown().
+    virtual void willShutdown();
+    // willDestroyIsolate() is called after disposing WorkerGlobalScope (and its
+    // WorkerScriptController) but before dispoing an Isolate.
     virtual void willDestroyIsolate();
     virtual void destroyIsolate();
     virtual void terminateV8Execution();