Add platform files for Background Sync API

public/platform/modules/background_sync/WebSyncClient.h

Index: public/platform/modules/background_sync/WebSyncClient.h
diff --git a/public/platform/modules/push_messaging/WebPushClient.h b/public/platform/modules/background_sync/WebSyncClient.h
similarity index 36%
copy from public/platform/modules/push_messaging/WebPushClient.h
copy to public/platform/modules/background_sync/WebSyncClient.h
index 09461099b18fd724a6b9ed7c41251d8d90bf18a2..402d1f8314ca80c5314a962449636f97f57a65f3 100644
--- a/public/platform/modules/push_messaging/WebPushClient.h
+++ b/public/platform/modules/background_sync/WebSyncClient.h
@@ -1,31 +1,29 @@
-// Copyright 2014 The Chromium Authors. All rights reserved.
+// Copyright 2015 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.
 
-#ifndef WebPushClient_h
-#define WebPushClient_h
+#ifndef WebSyncClient_h
+#define WebSyncClient_h
 
 #include "public/platform/WebCallbacks.h"
-#include "public/platform/modules/push_messaging/WebPushError.h"
+#include "public/platform/modules/background_sync/WebSyncError.h"
 
 namespace blink {
 
 class WebServiceWorkerRegistration;
-struct WebPushSubscription;
+struct WebSyncRegistration;
 
-using WebPushSubscriptionCallbacks = WebCallbacks<WebPushSubscription, WebPushError>;
-// FIXME: Remove when no longer used by the embedder - https://crbug.com/446883.
-using WebPushRegistrationCallbacks = WebPushSubscriptionCallbacks;
+using WebSyncRegistrationCallbacks = WebCallbacks<WebSyncRegistration, WebSyncError>;
 
-class WebPushClient {
+class WebSyncClient {
 public:
-    virtual ~WebPushClient() { }
+    virtual ~WebSyncClient() { }
 
     // Ownership of the WebServiceWorkerRegistration is not transferred.
     // Ownership of the callbacks is transferred to the client.
-    virtual void registerPushMessaging(WebServiceWorkerRegistration*, WebPushRegistrationCallbacks*) { BLINK_ASSERT_NOT_REACHED(); }
+    virtual void registerSync(WebServiceWorkerRegistration*, WebSyncRegistrationCallbacks*) = 0;
 };
 
 } // namespace blink
 
-#endif // WebPushClient_h
+#endif // WebSyncClient_h