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

public/platform/WebNotificationDelegate.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 WebNotificationDelegate_h
-#define WebNotificationDelegate_h
-
-namespace blink {
-
-// A delegate through which the embedder can trigger events on a Document-bound
-// Web Notifications object. Service Worker-bound Web Notifications will not have
-// a delegate, as their events will be fired on a Service Worker instead.
-class WebNotificationDelegate {
-public:
-    virtual void dispatchClickEvent() = 0;
-    virtual void dispatchShowEvent() = 0;
-    virtual void dispatchErrorEvent() = 0;
-    virtual void dispatchCloseEvent() = 0;
-};
-
-} // namespace blink
-
-#endif // WebNotificationDelegate_h
+#include "public/platform/modules/notifications/WebNotificationDelegate.h"