Make scrollable viewport size no longer depend on clip layer.

cc/layers/scrollbar_layer.cc

Index: cc/layers/scrollbar_layer.cc
diff --git a/cc/layers/scrollbar_layer.cc b/cc/layers/scrollbar_layer.cc
index 5f9e785121234b90c7bfa8a5f885cd96669ca5e6..a63b04bfe52488117805319c55e4b86514c900c5 100644
--- a/cc/layers/scrollbar_layer.cc
+++ b/cc/layers/scrollbar_layer.cc
@@ -137,6 +137,8 @@ void ScrollbarLayer::PushPropertiesTo(LayerImpl* layer) {
     scrollbar_layer->set_thumb_resource_id(thumb_->texture()->resource_id());
   else
     scrollbar_layer->set_thumb_resource_id(0);
+
+  scrollbar_layer->set_is_overlay_scrollbar(scrollbar_->IsOverlay());
 }
 
 ScrollbarLayer* ScrollbarLayer::ToScrollbarLayer() {