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..c80954ddc7dfda61b8a8c30a2d716fc59f0813af 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 <= 0, but they are
+    // managed by LayerTreeImpl and not LayerImpl, so should not be
+    // processed here.
+    if (!scrollbarLayer || (scrollbarLayer->scrollLayerId() <= 0))
         return;
 
     RawPtrLayerImplMap::const_iterator iter = newLayers.find(scrollbarLayer->id());