Make RootScroller set the outer viewport scroll layer in the compositor

cc/trees/layer_tree_impl.cc

Index: cc/trees/layer_tree_impl.cc
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index 04c375d598d431b69dba359b1b3d450aee220575..5f579cca568bdc0589a4d972ccfc7c6368911430 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -64,6 +64,7 @@ LayerTreeImpl::LayerTreeImpl(
       page_scale_layer_id_(Layer::INVALID_ID),
       inner_viewport_scroll_layer_id_(Layer::INVALID_ID),
       outer_viewport_scroll_layer_id_(Layer::INVALID_ID),
+      root_scroller_layer_id_(Layer::INVALID_ID),
       page_scale_factor_(page_scale_factor),
       min_page_scale_factor_(0),
       max_page_scale_factor_(0),
@@ -429,6 +430,8 @@ void LayerTreeImpl::PushPropertiesTo(LayerTreeImpl* target_tree) {
       overscroll_elasticity_layer_id_, page_scale_layer_id_,
       inner_viewport_scroll_layer_id_, outer_viewport_scroll_layer_id_);
 
+  target_tree->SetRootScrollerLayerFromId(root_scroller_layer_id_);
+
   target_tree->RegisterSelection(selection_);
 
   // This should match the property synchronization in
@@ -841,6 +844,10 @@ void LayerTreeImpl::SetViewportLayersFromIds(
   outer_viewport_scroll_layer_id_ = outer_viewport_scroll_layer_id;
 }
 
+void LayerTreeImpl::SetRootScrollerLayerFromId(int root_scroller_layer_id) {
+  root_scroller_layer_id_ = root_scroller_layer_id;
+}
+
 void LayerTreeImpl::ClearViewportLayers() {
   overscroll_elasticity_layer_id_ = Layer::INVALID_ID;
   page_scale_layer_id_ = Layer::INVALID_ID;