Move ScrollbarManager from core to platform and use as Scrollbar parent

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

-// 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.
-
-#include "core/paint/ScrollbarManager.h"
-
-namespace blink {
-
-ScrollbarManager::ScrollbarManager(ScrollableArea& scrollable_area)
-    : scrollable_area_(&scrollable_area),
-      h_bar_is_attached_(0),
-      v_bar_is_attached_(0) {}
-
-DEFINE_TRACE(ScrollbarManager) {
-  visitor->Trace(scrollable_area_);
-  visitor->Trace(h_bar_);
-  visitor->Trace(v_bar_);
-}
-
-void ScrollbarManager::Dispose() {
-  h_bar_is_attached_ = v_bar_is_attached_ = 0;
-  DestroyScrollbar(kHorizontalScrollbar);
-  DestroyScrollbar(kVerticalScrollbar);
-}
-
-}  // namespace blink