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

public/platform/WebPushRegistration.h

Index: public/platform/WebPushRegistration.h
diff --git a/public/platform/WebPushRegistration.h b/public/platform/WebPushRegistration.h
index b39e138896ab3a1c4c3f2294ab61098db54f335f..97e2afb0f49fbdc0200dfe39c2b5f7c26fbf9d2c 100644
--- a/public/platform/WebPushRegistration.h
+++ b/public/platform/WebPushRegistration.h
@@ -1,17 +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 WebPushRegistration_h
-#define WebPushRegistration_h
-
-#include "WebPushSubscription.h"
-
-namespace blink {
-
-// FIXME: Delete this file when no longer used - https://crbug.com/446883.
-using WebPushRegistration = WebPushSubscription;
-
-} // namespace blink
-
-#endif // WebPushRegistration_h
+#include "public/platform/modules/push_messaging/WebPushRegistration.h"