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

content/browser/service_worker/service_worker_url_request_job.cc

Index: content/browser/service_worker/service_worker_url_request_job.cc
diff --git a/content/browser/service_worker/service_worker_url_request_job.cc b/content/browser/service_worker/service_worker_url_request_job.cc
index 6a795d630de276a655d8a46490ff576385bed64f..29496ad3f9b04286b5c5cb7150cd3ce1ac93f549 100644
--- a/content/browser/service_worker/service_worker_url_request_job.cc
+++ b/content/browser/service_worker/service_worker_url_request_job.cc
@@ -229,6 +229,8 @@ void ServiceWorkerURLRequestJob::StartRequest() {
       fetch_dispatcher_.reset(new ServiceWorkerFetchDispatcher(
           CreateFetchRequest(),
           provider_host_->active_version(),
+          base::Bind(&ServiceWorkerURLRequestJob::DidPrepareFetchEvent,
+                     weak_factory_.GetWeakPtr()),
           base::Bind(&ServiceWorkerURLRequestJob::DidDispatchFetchEvent,
                      weak_factory_.GetWeakPtr())));
       fetch_dispatcher_->Run();
@@ -322,6 +324,11 @@ bool ServiceWorkerURLRequestJob::CreateRequestBodyBlob(std::string* blob_uuid,
   return true;
 }
 
+void ServiceWorkerURLRequestJob::DidPrepareFetchEvent() {
+  // TODO(shimazu): Set the timestamp to measure the time to launch SW
+  // This is related to this (http://crbug.com/401389)
+}
+
 void ServiceWorkerURLRequestJob::DidDispatchFetchEvent(
     ServiceWorkerStatusCode status,
     ServiceWorkerFetchEventResult fetch_result,