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 dfea08e082e103289c90e7c2cf39cca18c717e21..da142f9f797e7bd1ba89343ba99ed4379a87e959 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->child_at(i));
 
     ScrollbarLayerType* scrollbarLayer = layer->ToScrollbarLayer();
-    if (!scrollbarLayer)
+    // Pinch-zoom scrollbars will have an invalid scrollLayerId, but they are
+    // managed by LayerTreeImpl and not LayerImpl, so should not be
+    // processed here.
+    if (!scrollbarLayer || (scrollbarLayer->scroll_layer_id() == Layer::NOTUSED_ID))
         return;
 
     RawPtrLayerImplMap::const_iterator iter = newLayers.find(scrollbarLayer->id());