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

Source/core/rendering/compositing/RenderLayerCompositor.cpp

 /*
  * Copyright (C) 2009, 2010 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 958 matching lines @@
 
     m_renderView.frameView()->positionScrollbarLayers();
 }
 
 void RenderLayerCompositor::ensureRootLayer()
 {
     RootLayerAttachment expectedAttachment = m_renderView.frame()->isLocalRoot() ? RootLayerAttachedViaChromeClient : RootLayerAttachedViaEnclosingFrame;
     if (expectedAttachment == m_rootLayerAttachment)
          return;
 
+    Settings* settings = m_renderView.document().settings();
     if (!m_rootContentLayer) {
         m_rootContentLayer = GraphicsLayer::create(graphicsLayerFactory(), this);
         IntRect overflowRect = m_renderView.pixelSnappedLayoutOverflowRect();
         m_rootContentLayer->setSize(FloatSize(overflowRect.maxX(), overflowRect.maxY()));
         m_rootContentLayer->setPosition(FloatPoint());
         m_rootContentLayer->setOwnerNodeId(InspectorNodeIds::idForNode(m_renderView.generatingNode()));
 
-        // Need to clip to prevent transformed content showing outside this frame
-        m_rootContentLayer->setMasksToBounds(true);
+        // FIXME: with rootLayerScrolls, we probably don't even need m_rootContentLayer?
+        if (!(settings && settings->rootLayerScrolls())) {
+            // Need to clip to prevent transformed content showing outside this frame
+            m_rootContentLayer->setMasksToBounds(true);
+        }
     }
 
     if (!m_overflowControlsHostLayer) {
         ASSERT(!m_scrollLayer);
         ASSERT(!m_containerLayer);
 
         // Create a layer to host the clipping layer and the overflow controls layers.
         m_overflowControlsHostLayer = GraphicsLayer::create(graphicsLayerFactory(), this);
 
         // Clip iframe's overflow controls layer.
@@ skipping 191 matching lines @@
     } else if (graphicsLayer == m_scrollLayer.get()) {
         name = "LocalFrame Scrolling Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace blink