ServiceWorker: Add prepare_callback in DispatchFetchEvent to notify the end of SW preparation.

content/browser/service_worker/service_worker_version.h

Index: content/browser/service_worker/service_worker_version.h
diff --git a/content/browser/service_worker/service_worker_version.h b/content/browser/service_worker/service_worker_version.h
index 48cbda98f59a41640da1c5e0974d4b9468920fc2..74dbb391fe7802a9cc9f1044814931d150bf1fcd 100644
--- a/content/browser/service_worker/service_worker_version.h
+++ b/content/browser/service_worker/service_worker_version.h
@@ -180,7 +180,8 @@ class CONTENT_EXPORT ServiceWorkerVersion
   // This must be called when the status() is ACTIVATED. Calling this in other
   // statuses will result in an error SERVICE_WORKER_ERROR_FAILED.
   void DispatchFetchEvent(const ServiceWorkerFetchRequest& request,
-                          const FetchCallback& callback);
+                          const base::Closure& prepare_callback,
+                          const FetchCallback& fetch_callback);
 
   // Sends sync event to the associated embedded worker and asynchronously calls
   // |callback| when it errors out or it gets response from the worker to notify