Adds UMA for screen rotation animation smoothness.

ash/rotator/screen_rotation_animator.h

Index: ash/rotator/screen_rotation_animator.h
diff --git a/ash/rotator/screen_rotation_animator.h b/ash/rotator/screen_rotation_animator.h
index ac6f18a83569e0e1aeab745949ba9149bf5342fd..2928a575c90297cb1406c36c9e2e7243ea65a1ad 100644
--- a/ash/rotator/screen_rotation_animator.h
+++ b/ash/rotator/screen_rotation_animator.h
@@ -56,6 +56,7 @@ class ASH_EXPORT ScreenRotationAnimator {
  private:
   friend class ash::test::ScreenRotationAnimatorTestApi;
   struct ScreenRotationRequest;
+  class ScreenRotationAnimationMetricsReporter;
 
   // Set the screen orientation to |new_rotation| and animate the change. The
   // animation will rotate the initial orientation's layer towards the new
@@ -82,6 +83,7 @@ class ASH_EXPORT ScreenRotationAnimator {
       screen_rotation_animator_observers_;
   std::unique_ptr<ui::LayerTreeOwner> old_layer_tree_owner_;
   std::unique_ptr<ScreenRotationRequest> last_pending_request_;
+  std::unique_ptr<ScreenRotationAnimationMetricsReporter> metrics_reporter_;
   base::WeakPtrFactory<ScreenRotationAnimator> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ScreenRotationAnimator);