Allow proper highlighting on universal overflow scroll.

Source/core/rendering/compositing/GraphicsLayerTreeBuilder.cpp

 /*
  * Copyright (C) 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2014 Google 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
@@ skipping 67 matching lines @@
     if (layer.stackingNode()->isStackingContext()) {
         RenderLayerStackingNodeIterator iterator(*layer.stackingNode(), NegativeZOrderChildren);
         while (RenderLayerStackingNode* curNode = iterator.next())
             rebuild(*curNode->layer(), childList);
 
         // If a negative z-order child is compositing, we get a foreground layer which needs to get parented.
         if (hasCompositedLayerMapping && currentCompositedLayerMapping->foregroundLayer())
             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);
 
     if (hasCompositedLayerMapping) {
         bool parented = false;
         if (layer.renderer()->isRenderPart())
             parented = RenderLayerCompositor::parentFrameContentLayers(toRenderPart(layer.renderer()));
 
         if (!parented)
@@ skipping 17 matching lines @@
                 currentCompositedLayerMapping->parentForSublayers()->addChild(overflowControlLayer);
             }
         }
 
         if (shouldAppendLayer(layer))
             childLayersOfEnclosingLayer.append(currentCompositedLayerMapping->childForSuperlayers());
     }
 }
 
 }