CC Animations: Port Impl-only-scrolling to use compositor animation timelines.

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 35151a7d4e43363c73c0312c1a335bdcff4c74c8..f32c22ea60ebaeea7c2db1bdaf615a5b50075afe 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -1373,6 +1373,13 @@ void LayerTreeHost::SetLayerScrollOffsetMutated(
   layer->OnScrollOffsetAnimated(scroll_offset);
 }
 
+gfx::ScrollOffset LayerTreeHost::GetScrollOffsetForAnimation(
+    int layer_id) const {
+  LayerAnimationValueProvider* layer = LayerById(layer_id);
+  DCHECK(layer);
+  return layer->ScrollOffsetForAnimation();
+}
+
 bool LayerTreeHost::IsAnimatingFilterProperty(const Layer* layer) const {
   return animation_host_
              ? animation_host_->IsAnimatingFilterProperty(layer->id())