Implement pinch-zoom scaling for main-frame scrollbars and pinch-zoom overlay scrollbars.

cc/tree_synchronizer.cc

Index: cc/tree_synchronizer.cc
diff --git a/cc/tree_synchronizer.cc b/cc/tree_synchronizer.cc
index 8c3a3700a8cdc31bb350fbfe69d1e1564709c356..16af96f92cdccfb9fd03fe80910601d8bd64ad82 100644
--- a/cc/tree_synchronizer.cc
+++ b/cc/tree_synchronizer.cc
@@ -115,7 +115,10 @@ void updateScrollbarLayerPointersRecursiveInternal(const RawPtrLayerImplMap& new
         updateScrollbarLayerPointersRecursiveInternal<LayerType, ScrollbarLayerType>(newLayers, layer->childAt(i));
 
     ScrollbarLayerType* scrollbarLayer = layer->toScrollbarLayer();
-    if (!scrollbarLayer)
+    // Pinch-zoom scrollbars will have a scrollLayerId of -1, but they are

[enne (OOO), 2013/03/06 19:32:58]

Can you add a named constant to cc::Layer (since it manages layer ids) instead
of just using -1?

[wjmaclean, 2013/03/06 22:36:53]

Done.

+    // managed by LayerTreeImpl and not LayerImpl, so should not be
+    // processed here.
+    if (!scrollbarLayer || (scrollbarLayer->scrollLayerId() == -1))
         return;
 
     RawPtrLayerImplMap::const_iterator iter = newLayers.find(scrollbarLayer->id());