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

cc/trees/mutator_host_client.h

Index: cc/trees/mutator_host_client.h
diff --git a/cc/trees/mutator_host_client.h b/cc/trees/mutator_host_client.h
index ffb2329e7ccad73f3e579e2c52163941b5786e45..56123b4a62a81554c7544889f3ec6c3899bff0e3 100644
--- a/cc/trees/mutator_host_client.h
+++ b/cc/trees/mutator_host_client.h
@@ -35,6 +35,9 @@ class MutatorHostClient {
       int layer_id,
       LayerTreeType tree_type,
       const gfx::ScrollOffset& scroll_offset) = 0;
+
+  virtual void ScrollOffsetAnimationFinished() = 0;
+  virtual gfx::ScrollOffset GetScrollOffsetForAnimation(int layer_id) const = 0;
 };
 
 }  // namespace cc