Make RootScroller set the outer viewport scroll layer in the compositor

cc/trees/layer_tree_host.cc

Index: cc/trees/layer_tree_host.cc
diff --git a/cc/trees/layer_tree_host.cc b/cc/trees/layer_tree_host.cc
index 07940b38bc48f51c0c42e55953182fd09c19dacf..88b132d3f57a5438aedeb5882bf2b97bfd6d5187 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -491,6 +491,10 @@ void LayerTreeHost::FinishCommitOnImplThread(LayerTreeHostImpl* host_impl) {
     sync_tree->ClearViewportLayers();
   }
 
+  sync_tree->SetRootScrollerLayerFromId(root_scroller_layer_.get()
+                                            ? root_scroller_layer_->id()
+                                            : Layer::INVALID_ID);
+
   sync_tree->RegisterSelection(selection_);
 
   bool property_trees_changed_on_active_tree =
@@ -1216,6 +1220,11 @@ void LayerTreeHost::RegisterViewportLayers(
   outer_viewport_scroll_layer_ = outer_viewport_scroll_layer;
 }
 
+void LayerTreeHost::SetRootScrollerLayer(
+    scoped_refptr<Layer> root_scroller_layer) {
+  root_scroller_layer_ = root_scroller_layer;
+}
+
 void LayerTreeHost::RegisterSelection(const LayerSelection& selection) {
   if (selection_ == selection)
     return;