Move ScrollbarManager from core to platform and use as Scrollbar parent

third_party/WebKit/Source/core/paint/ScrollbarManager.h

-// Copyright 2016 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 ScrollbarManager_h
-#define ScrollbarManager_h
-
-#include "core/CoreExport.h"
-#include "platform/scroll/ScrollableArea.h"
-
-namespace blink {
-
-class CORE_EXPORT ScrollbarManager {
-  DISALLOW_NEW();
-
-  // Helper class to manage the life cycle of Scrollbar objects.
- public:
-  ScrollbarManager(ScrollableArea&);
-
-  void Dispose();
-
-  Scrollbar* HorizontalScrollbar() const {
-    return h_bar_is_attached_ ? h_bar_.Get() : nullptr;
-  }
-  Scrollbar* VerticalScrollbar() const {
-    return v_bar_is_attached_ ? v_bar_.Get() : nullptr;
-  }
-  bool HasHorizontalScrollbar() const { return HorizontalScrollbar(); }
-  bool HasVerticalScrollbar() const { return VerticalScrollbar(); }
-
-  // These functions are used to create/destroy scrollbars.
-  virtual void SetHasHorizontalScrollbar(bool has_scrollbar) = 0;
-  virtual void SetHasVerticalScrollbar(bool has_scrollbar) = 0;
-
-  DECLARE_VIRTUAL_TRACE();
-
- protected:
-  // TODO(ymalik): This can be made non-virtual since there's a lot of
-  // common code in subclasses.
-  virtual Scrollbar* CreateScrollbar(ScrollbarOrientation) = 0;
-  virtual void DestroyScrollbar(ScrollbarOrientation) = 0;
-
- protected:
-  Member<ScrollableArea> scrollable_area_;
-
-  // The scrollbars associated with m_scrollableArea. Both can nullptr.
-  Member<Scrollbar> h_bar_;
-  Member<Scrollbar> v_bar_;
-
-  unsigned h_bar_is_attached_ : 1;
-  unsigned v_bar_is_attached_ : 1;
-};
-
-}  // namespace blink
-
-#endif  // ScrollbarManager_h