Update from https://crrev.com/312398

cc/animation/animation.h

Index: cc/animation/animation.h
diff --git a/cc/animation/animation.h b/cc/animation/animation.h
index 80c0763929ba046f21a265848a6fd17977dc3cd3..9342f1abc12bf880e624685ffa427c9a55944708 100644
--- a/cc/animation/animation.h
+++ b/cc/animation/animation.h
@@ -174,14 +174,13 @@ class CC_EXPORT Animation {
 
   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_;