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 ecae10d776a58d792f39b737540620c6fdc49fb5..4f511de8e216d69f8e57ce8d21d3698621e1ed15 100644
--- a/cc/tree_synchronizer.cc
+++ b/cc/tree_synchronizer.cc
@@ -148,7 +148,11 @@ void UpdateScrollbarLayerPointersRecursiveInternal(
   }
 
   ScrollbarLayerType* scrollbar_layer = layer->ToScrollbarLayer();
-  if (!scrollbar_layer)
+  // Pinch-zoom scrollbars will have an invalid scrollLayerId, but they are
+  // managed by LayerTreeImpl and not LayerImpl, so should not be
+  // processed here.
+  if (!scrollbar_layer || (scrollbar_layer->scroll_layer_id() ==
+                          Layer::PINCH_ZOOM_ROOT_SCROLL_LAYER_ID))
     return;
 
   RawPtrLayerImplMap::const_iterator iter =