Add platform files for Background Sync API

public/platform/modules/background_sync/WebSyncClient.h

+// 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 WebSyncClient_h
+#define WebSyncClient_h
+
+#include "public/platform/WebCallbacks.h"
+#include "public/platform/modules/background_sync/WebSyncError.h"
+
+namespace blink {
+
+class WebServiceWorkerRegistration;
+struct WebSyncRegistration;
+
+using WebSyncRegistrationCallbacks = WebCallbacks<WebSyncRegistration, WebSyncError>;
+
+class WebSyncClient {
+public:
+    virtual ~WebSyncClient() { }
+
+    // Ownership of the WebServiceWorkerRegistration is not transferred.
+    // Ownership of the callbacks is transferred to the client.
+    virtual void registerSync(WebServiceWorkerRegistration*, WebSyncRegistrationCallbacks*) { BLINK_ASSERT_NOT_REACHED(); }
+};
+
+} // namespace blink
+
+#endif // WebSyncClient_h