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

public/platform/WebPushClient.h

-// 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 WebPushClient_h
-#define WebPushClient_h
-
-#include "public/platform/WebCallbacks.h"
-#include "public/platform/WebPushError.h"
-
-namespace blink {
-
-class WebServiceWorkerRegistration;
-struct WebPushSubscription;
-
-using WebPushSubscriptionCallbacks = WebCallbacks<WebPushSubscription, WebPushError>;
-// FIXME: Remove when no longer used by the embedder - https://crbug.com/446883.
-using WebPushRegistrationCallbacks = WebPushSubscriptionCallbacks;
-
-class WebPushClient {
-public:
-    virtual ~WebPushClient() { }
-
-    // Ownership of the WebServiceWorkerRegistration is not transferred.
-    // Ownership of the callbacks is transferred to the client.
-    virtual void registerPushMessaging(WebServiceWorkerRegistration*, WebPushRegistrationCallbacks*) { BLINK_ASSERT_NOT_REACHED(); }
-};
-
-} // namespace blink
-
-#endif // WebPushClient_h
+#include "public/platform/modules/push_messaging/WebPushClient.h"