Allow proper highlighting on universal overflow scroll.

Source/core/rendering/compositing/GraphicsLayerTreeBuilder.cpp

Index: Source/core/rendering/compositing/GraphicsLayerTreeBuilder.cpp
diff --git a/Source/core/rendering/compositing/GraphicsLayerTreeBuilder.cpp b/Source/core/rendering/compositing/GraphicsLayerTreeBuilder.cpp
index 9826be1185ed18813e8972be622b3a63ceac7416..1188aed30df4f00a05bff16924dbbcd4fcffe59e 100644
--- a/Source/core/rendering/compositing/GraphicsLayerTreeBuilder.cpp
+++ b/Source/core/rendering/compositing/GraphicsLayerTreeBuilder.cpp
@@ -85,6 +85,9 @@ void GraphicsLayerTreeBuilder::rebuild(RenderLayer& layer, GraphicsLayerVector&
             childList.append(currentCompositedLayerMapping->foregroundLayer());
     }
 
+    if (hasCompositedLayerMapping && currentCompositedLayerMapping->scrollingBlockSelectionLayer())

[Ian Vollick, 2014/06/12 14:54:57]

Please add a comment here explaining why this code (which looks like it ought to
be in CLM) really must be here.

+        childList.append(currentCompositedLayerMapping->scrollingBlockSelectionLayer());

[abarth-chromium, 2014/06/13 04:12:02]

Note: If you have this logic here, you need to schedule a
CompositingUpdateRebuildTree to make sure we actually rebuild the tree.

[hartmanng, 2014/06/13 14:51:38]

Turns out we can get away without this part by fixing up CLM a bit. I think it's
a better solution anyway, so I don't need to touch GraphcisLayerTreeBuilder
anymore.

+
     RenderLayerStackingNodeIterator iterator(*layer.stackingNode(), NormalFlowChildren | PositiveZOrderChildren);
     while (RenderLayerStackingNode* curNode = iterator.next())
         rebuild(*curNode->layer(), childList);