Revert "MainFrame scrollbars should work with RFV instead of FV"

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& scrollableArea)
     : m_scrollableArea(&scrollableArea),
       m_hBarIsAttached(0),
       m_vBarIsAttached(0) {}
 
 DEFINE_TRACE(ScrollbarManager) {
   visitor->trace(m_scrollableArea);
   visitor->trace(m_hBar);
   visitor->trace(m_vBar);
 }
 
-void ScrollbarManager::setScroller(ScrollableArea* scroller) {
-  DCHECK(m_scrollableArea != scroller);
-  DCHECK(scroller);
-  DCHECK(scroller->isRootFrameViewport() || scroller->isFrameView() ||
-         scroller->isPaintLayerScrollableArea());
-  dispose();
-  m_scrollableArea = scroller;
-}
-
 void ScrollbarManager::dispose() {
   m_hBarIsAttached = m_vBarIsAttached = 0;
   destroyScrollbar(HorizontalScrollbar);
   destroyScrollbar(VerticalScrollbar);
 }
 
 }  // namespace blink