Push API: rename registration to subscription.

public/platform/WebPushSubscription.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 WebPushRegistration_h
-#define WebPushRegistration_h
+#ifndef WebPushSubscription_h
+#define WebPushSubscription_h
 
 #include "WebString.h"
 
 namespace blink {
 
-struct WebPushRegistration {
-    WebPushRegistration(const WebString& endpoint, const WebString& registrationId)
+struct WebPushSubscription {
+    WebPushSubscription(const WebString& endpoint, const WebString& subscriptionId)
         : endpoint(endpoint)
-        , registrationId(registrationId)
+        , subscriptionId(subscriptionId)
     {
     }
 
     WebString endpoint;
-    WebString registrationId;
+    WebString subscriptionId;
 };
 
 } // namespace blink
 
-#endif // WebPushRegistration_h
+#endif // WebPushSubscription_h