Move Push-related Blink API headers into their own directory.

public/platform/WebPushSubscription.h

Index: public/platform/WebPushSubscription.h
diff --git a/public/platform/WebPushSubscription.h b/public/platform/WebPushSubscription.h
index 51528a3951624c98d3b26f1a523da0b45f208983..c2fa5b9c0164ff5d8a4719c75f2ced8827667205 100644
--- a/public/platform/WebPushSubscription.h
+++ b/public/platform/WebPushSubscription.h
@@ -1,25 +1 @@
-// Copyright 2014 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 WebPushSubscription_h
-#define WebPushSubscription_h
-
-#include "WebString.h"
-
-namespace blink {
-
-struct WebPushSubscription {
-    WebPushSubscription(const WebString& endpoint, const WebString& subscriptionId)
-        : endpoint(endpoint)
-        , subscriptionId(subscriptionId)
-    {
-    }
-
-    WebString endpoint;
-    WebString subscriptionId;
-};
-
-} // namespace blink
-
-#endif // WebPushSubscription_h
+#include "public/platform/modules/push_messaging/WebPushSubscription.h"