Let LayerTreeHostImpl::ScrollAnimated scroll the outer viewport.

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 a77e5179c170e86d3f82398b957bfe05dca45cbb..fc75de6839f4158698df583161b121eb42e12bfd 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -365,8 +365,6 @@ LayerImpl* LayerTreeImpl::CurrentlyScrollingLayer() const {
 }
 
 void LayerTreeImpl::SetCurrentlyScrollingLayer(LayerImpl* layer) {
-  DCHECK_IMPLIES(layer, layer != OuterViewportScrollLayer());
-
   int new_id = layer ? layer->id() : Layer::INVALID_ID;
   if (currently_scrolling_layer_id_ == new_id)
     return;