Fix PaintPropertyTreeBuilder for root layer scrolling.

third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp

Index: third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
index 6248097b19031e13208183b785a026fa7d0538de..f887e290dd1141a729945c551700aecd4152ae57 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
@@ -1136,7 +1136,7 @@ void CompositedLayerMapping::updateInternalHierarchy()
 
     // Now constructing the subtree for the overflow controls.
     bottomLayer = m_graphicsLayer.get();
-    if (m_isMainFrameLayoutViewLayer)
+    if (m_isMainFrameLayoutViewLayer && !RuntimeEnabledFeatures::slimmingPaintV2Enabled())

[skobes, 2016/08/17 01:38:24]

This still makes me nervous but if the paint team is ok with it then *shrug*.

[pdr., 2016/08/17 04:30:41]

I agree but we just aren't far enough along to block you on this :(

I've filed https://crbug.com/638473 for this work.

[trchen, 2016/08/17 05:57:47]

I think this line is totally fine. The long term goal is to bypass
core/layout/compositing. Today we are still using PaintLayerCompositor just
because it is a pain to duplicate some basic setup. If possible I don't even
want LayoutView to create CLM... Because it pops main frame scrollbars to a
graphics layer.

         bottomLayer = layoutObject()->frame()->page()->frameHost().visualViewport().containerLayer();
     updateBottomLayer(m_overflowControlsAncestorClippingLayer.get());
     updateBottomLayer(m_overflowControlsHostLayer.get());