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 f0236dd330f12cad9055f49b69373884e2044da9..918d5db58fc8c1d738e4e025f2985a9b896f10b9 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -124,7 +124,7 @@ LayerTreeHost::LayerTreeHost(InitParams* params)
 
   if (settings_.accelerated_animation_enabled) {
     if (settings_.use_compositor_animation_timelines) {
-      animation_host_ = AnimationHost::Create();
+      animation_host_ = AnimationHost::Create(ThreadInstance::MAIN);
       animation_host_->SetMutatorHostClient(this);
     } else {
       animation_registrar_ = AnimationRegistrar::Create();
@@ -1149,6 +1149,13 @@ 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::ScrollOffsetAnimationWasInterrupted(
     const Layer* layer) const {
   return animation_host_