cc: Enable composited border-radius 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 c438cf02eb89b27ee507e657c12a2d6998906d3c..6b4c3dd8b4220b30cf3152ee6372a0cefe0b4f4a 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
@@ -591,9 +591,6 @@ void CompositedLayerMapping::
   if (!clipping_container)
     return;
 
-  if (clipping_container->EnclosingLayer() == scroll_parent)
-    return;
-
   if (compositing_ancestor->GetLayoutObject().IsDescendantOf(
           clipping_container))
     return;
@@ -624,6 +621,9 @@ void CompositedLayerMapping::
       owning_layer_is_clipped &&
       clipping_container->Style()->HasBorderRadius() &&
       !AncestorRoundedCornersWontClip(GetLayoutObject(), *clipping_container);
+
+  if (clipping_container->EnclosingLayer() == scroll_parent)
+    owning_layer_is_clipped = owning_layer_is_masked;

[trchen, 2017/05/22 23:00:06]

Could you document what does this line do? Actually I don't understand this
either before your CL or after your CL.

[sunxd, 2017/06/15 15:59:10]

Sure. It was added due to this bug: crbug.com/450756.
I think it's trying to avoid applying scroll parent as the ancestor clipping
layer to composited scrollers.

[Stephen Chennney, 2017/06/15 17:44:24]

Will we not now have the same problem as crbug.com/450756 in cases where the
mask is required? Could you make a test based off
compositing/overflow/avoid-ancestor-clip-for-scroll-children.html that has a
border radius to invoke the mask?

[sunxd, 2017/06/15 19:59:20]

Oh sorry I think this change should be reverted as the new fix is going to take
care of the mask problem of scroll_parents.

 }
 
 const PaintLayer* CompositedLayerMapping::ScrollParent() {