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

cc/scrollbar_layer.cc

Index: cc/scrollbar_layer.cc
diff --git a/cc/scrollbar_layer.cc b/cc/scrollbar_layer.cc
index 6d8d6ea6cbe91b71f2274b0ef13ea2bbe374dd3e..9ca1a8c609c0fefab7a0439e01dd6f25a06845e0 100644
--- a/cc/scrollbar_layer.cc
+++ b/cc/scrollbar_layer.cc
@@ -129,6 +129,11 @@ void ScrollbarLayer::PushPropertiesTo(LayerImpl* layer) {
     scrollbar_layer->set_thumb_resource_id(thumb_->texture()->resourceId());
   else
     scrollbar_layer->set_thumb_resource_id(0);
+
+  // Pinch zoom scrollbarLayerImpl does not get its scroll_layer_id_
+  // set in LayerImpl, so we need to push it here.
+  if (scroll_layer_id_ == Layer::PINCH_ZOOM_ROOT_SCROLL_LAYER_ID)
+    scrollbar_layer->set_scroll_layer_id(scroll_layer_id_);
 }
 
 ScrollbarLayer* ScrollbarLayer::ToScrollbarLayer() {