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

Source/core/rendering/compositing/RenderLayerCompositor.cpp

Index: Source/core/rendering/compositing/RenderLayerCompositor.cpp
diff --git a/Source/core/rendering/compositing/RenderLayerCompositor.cpp b/Source/core/rendering/compositing/RenderLayerCompositor.cpp
index 95204d6cc37f3a1614d8ca9eb6273478237754e3..009eadc5e0be10c3d5bc748ac82fe64d875d3241 100644
--- a/Source/core/rendering/compositing/RenderLayerCompositor.cpp
+++ b/Source/core/rendering/compositing/RenderLayerCompositor.cpp
@@ -976,6 +976,7 @@ void RenderLayerCompositor::ensureRootLayer()
     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();
@@ -983,8 +984,11 @@ void RenderLayerCompositor::ensureRootLayer()
         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) {