[ServiceWorker] Convert ping-pong IPC into mojo interface ServiceWorkerEventDispatcher

content/common/service_worker/service_worker_event_dispatcher.mojom

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 module content.mojom;
 
 import "components/payments/content/payment_app.mojom";
 import "content/common/url_loader.mojom";
 import "mojo/common/string16.mojom";
 import "mojo/common/time.mojom";
@@ skipping 55 matching lines @@
   DispatchSyncEvent(string tag,
                     blink.mojom.BackgroundSyncEventLastChance last_chance)
       => (blink.mojom.ServiceWorkerEventStatus status,
           mojo.common.mojom.Time dispatch_event_time);
   DispatchPaymentRequestEvent(payments.mojom.PaymentAppRequest app_request)
       => (blink.mojom.ServiceWorkerEventStatus status,
           mojo.common.mojom.Time dispatch_event_time);
   DispatchExtendableMessageEvent(ExtendableMessageEvent event)
       => (blink.mojom.ServiceWorkerEventStatus status,
           mojo.common.mojom.Time dispatch_event_time);
+
+  // Pings the service worker to check if it is responsive. If the callback is
+  // not called within a certain period of time, the browser will terminate the
+  // worker. Unlike the other functions in this interface, Ping() does not
+  // dispatch an event.
+  Ping() => ();

[falken, 2017/03/22 05:46:30]

Just to be sure, we are guaranteed Ping() is called on the service worker
thread, right?

[leonhsl(Using Gerrit), 2017/03/22 06:45:59]

From my current knowledge, in browser side SWVersion calls this mojo function
Ping() on IO thread, and in renderer side SWContextClient executes its impl of
Ping() on its own service worker thread. Hopefully I'm understanding correctly
:)

[shimazu, 2017/03/22 06:46:26]

Yeah, ServiceWorkerEventDispather is bound after the worker thread has started.

 };