Update from https://crrev.com/312398

cc/animation/animation.h

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_ANIMATION_ANIMATION_H_
 #define CC_ANIMATION_ANIMATION_H_
 
 #include "base/basictypes.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/time/time.h"
@@ skipping 156 matching lines @@
  private:
   Animation(scoped_ptr<AnimationCurve> curve,
             int animation_id,
             int group_id,
             TargetProperty target_property);
 
   base::TimeDelta ConvertToActiveTime(base::TimeTicks monotonic_time) const;
 
   scoped_ptr<AnimationCurve> curve_;
 
-  // IDs are not necessarily unique.
+  // IDs must be unique.
   int id_;
 
   // Animations that must be run together are called 'grouped' and have the same
   // group id. Grouped animations are guaranteed to start at the same time and
   // no other animations may animate any of the group's target properties until
-  // all animations in the group have finished animating. Note: an active
-  // animation's group id and target property uniquely identify that animation.
+  // all animations in the group have finished animating.
   int group_;
 
   TargetProperty target_property_;
   RunState run_state_;
   double iterations_;
   double iteration_start_;
   base::TimeTicks start_time_;
   Direction direction_;
   double playback_rate_;
   FillMode fill_mode_;
@@ skipping 43 matching lines @@
   // longer affect any observers, and are deleted.
   bool affects_active_observers_;
   bool affects_pending_observers_;
 
   DISALLOW_COPY_AND_ASSIGN(Animation);
 };
 
 }  // namespace cc
 
 #endif  // CC_ANIMATION_ANIMATION_H_