Add platform files for Background Sync API

public/platform/modules/background_sync/WebSyncError.h

-// 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 WebPushError_h
-#define WebPushError_h
+#ifndef WebSyncError_h
+#define WebSyncError_h
 
 #include "public/platform/WebString.h"
 
 namespace blink {
 
-struct WebPushError {
+struct WebSyncError {
     enum ErrorType {
         ErrorTypeAbort = 0,
-        ErrorTypeNetwork,
+        ErrorTypeNoPermission,
         ErrorTypeNotFound,
         ErrorTypeUnknown,
         ErrorTypeLast = ErrorTypeUnknown
     };
 
-    WebPushError(ErrorType errorType, const WebString& message)
+    WebSyncError(ErrorType errorType, const WebString& message)
         : errorType(errorType)
         , message(message)
     {
     }
 
     ErrorType errorType;
     WebString message;
 };
 
 } // namespace blink
 
-#endif // WebPushError_h
+#endif // WebSyncError_h