Add a setting to do frame scrolls through the root layer.

Source/core/paint/BlockPainter.cpp

 // 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.
 
 #include "config.h"
 #include "core/paint/BlockPainter.h"
 
 #include "core/editing/Caret.h"
 #include "core/editing/FrameSelection.h"
 #include "core/frame/LocalFrame.h"
@@ skipping 52 matching lines @@
             && m_renderBlock.firstChild() && m_renderBlock.lastChild() && m_renderBlock.firstChild() != m_renderBlock.lastChild();
         if (shouldCull)
             cullSaver.cull(overflowBox);
 
         m_renderBlock.paintObject(paintInfo, adjustedPaintOffset);
     }
 
     // Our scrollbar widgets paint exactly when we tell them to, so that they work properly with
     // z-index. We paint after we painted the background/border, so that the scrollbars will
     // sit above the background/border.
+    paintOverflowControlsIfNeeded(paintInfo, adjustedPaintOffset);
+}
+
+void BlockPainter::paintOverflowControlsIfNeeded(PaintInfo& paintInfo, const LayoutPoint& paintOffset)
+{
+    PaintPhase phase = paintInfo.phase;
     if (m_renderBlock.hasOverflowClip() && m_renderBlock.style()->visibility() == VISIBLE && (phase == PaintPhaseBlockBackground || phase == PaintPhaseChildBlockBackground) && paintInfo.shouldPaintWithinRoot(&m_renderBlock) && !paintInfo.paintRootBackgroundOnly()) {
-        DrawingRecorder recorder(paintInfo.context, &m_renderBlock, paintInfo.phase, pixelSnappedIntRect(adjustedPaintOffset, m_renderBlock.visualOverflowRect().size()));
-        m_renderBlock.layer()->scrollableArea()->paintOverflowControls(paintInfo.context, roundedIntPoint(adjustedPaintOffset), paintInfo.rect, false /* paintingOverlayControls */);
+        DrawingRecorder recorder(paintInfo.context, &m_renderBlock, paintInfo.phase, pixelSnappedIntRect(paintOffset, m_renderBlock.visualOverflowRect().size()));
+        m_renderBlock.layer()->scrollableArea()->paintOverflowControls(paintInfo.context, roundedIntPoint(paintOffset), paintInfo.rect, false /* paintingOverlayControls */);
     }
 }
 
 void BlockPainter::paintChildren(PaintInfo& paintInfo, const LayoutPoint& paintOffset)
 {
     for (RenderBox* child = m_renderBlock.firstChildBox(); child; child = child->nextSiblingBox())
         paintChild(child, paintInfo, paintOffset);
 }
 
 void BlockPainter::paintChild(RenderBox* child, PaintInfo& paintInfo, const LayoutPoint& paintOffset)
@@ skipping 427 matching lines @@
         RenderBlock* block = flow->containingBlock();
         for ( ; block && block != &m_renderBlock; block = block->containingBlock())
             accumulatedPaintOffset.moveBy(block->location());
         ASSERT(block);
         InlinePainter(*flow).paintOutline(info, accumulatedPaintOffset);
     }
 }
 
 
 } // namespace blink