scheduler: Rename WebPageScheduler to WebFrameHostScheduler

public/platform/WebPageScheduler.h

-// 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 WebPageScheduler_h
-#define WebPageScheduler_h
-
-#include "WebCommon.h"
-
-namespace blink {
-
-class WebFrameScheduler;
-
-class BLINK_PLATFORM_EXPORT WebPageScheduler {
-public:
-    virtual ~WebPageScheduler() { }
-
-    // The scheduler may throttle tasks associated with backgound pages.
-    virtual void setPageInBackground(bool) { }
-
-    // Creaters a new WebFrameScheduler, the caller is responsible for deleting it.
-    WebFrameScheduler* createFrameScheduler() { return nullptr; }
-};
-
-} // namespace blink
-
-#endif // WebPageScheduler_h