ServiceWorker: Introduce ExtendableMessageEvent to replace MessageEvent

public/web/WebServiceWorkerContextClient.h

Index: public/web/WebServiceWorkerContextClient.h
diff --git a/public/web/WebServiceWorkerContextClient.h b/public/web/WebServiceWorkerContextClient.h
index d144efa7f634893d0f79066940f85cc1bf0dff6b..376284c95792cca0d8f0bd01b4e6298467dae1f7 100644
--- a/public/web/WebServiceWorkerContextClient.h
+++ b/public/web/WebServiceWorkerContextClient.h
@@ -141,6 +141,11 @@ public:
     // ServiceWorker's script context.
     virtual void didHandleCrossOriginConnectEvent(int connectEventID, bool acceptConnect) { }
 
+    // ServiceWorker specific method. Called after ExtendableMessageEvent
+    // (dispatched via WebServiceWorkerContextProxy) is handled by the
+    // ServiceWorker's script context.
+    virtual void didHandleMessageEvent(int messageEventID, WebServiceWorkerEventResult result) {}
+
     // Ownership of the returned object is transferred to the caller.
     // This is called on the main thread.
     virtual WebServiceWorkerNetworkProvider* createServiceWorkerNetworkProvider(WebDataSource*) { return nullptr; }