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..c477253772a3da1157e9477fb1932c31e2399ab5 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
@@ -76,6 +77,7 @@ class ASH_EXPORT ScreenRotationAnimator {
   // The id of the display to rotate.
   int64_t display_id_;
   bool is_rotating_;
+  std::unique_ptr<ScreenRotationAnimationMetricsReporter> metrics_reporter_;
   // Only set in unittest to disable animation timers.
   bool disable_animation_timers_for_test_;
   base::ObserverList<ScreenRotationAnimatorObserver>