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 9debbf9ae796fa61f9d4a7996f1c38efb4452ff1..e120a7d37d03504914b30da35d95314b27e61f45 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() {