Bug 1518374 - Remove obsolete comment in Animation.h about kNoIndex; r=hiro

As of bug 1203009 / changeset 37b6deedaab6 and per spec, we no longer reset the
animation index when an Animation re-enters the idle state and kNoIndex is no
longer defined.

Depends on D15898

Differential Revision: https://phabricator.services.mozilla.com/D15899

--HG--
extra : moz-landing-system : lando

dom/animation/Animation.h

@@ -536,8 +536,6 @@ class Animation : public DOMEventTargetHelper,
   static uint64_t sNextAnimationIndex;
 
   // The relative position of this animation within the global animation list.
-  // This is kNoIndex while the animation is in the idle state and is updated
-  // each time the animation transitions out of the idle state.
   //
   // Note that subclasses such as CSSTransition and CSSAnimation may repurpose
   // this member to implement their own brand of sorting. As a result, it is