Web Animations: Compositor timeOffset should be sensitive to playback rate

Source/core/animation/AnimationPlayer.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 605 matching lines @@
 }
 
 bool AnimationPlayer::maybeStartAnimationOnCompositor()
 {
     if (!canStartAnimationOnCompositor())
         return false;
 
     double startTime = timeline()->zeroTime() + startTimeInternal();
     double timeOffset = 0;
     if (std::isnan(startTime)) {
-        timeOffset = currentTimeInternal();
+        timeOffset = m_playbackRate < 0 ? sourceEnd() - currentTimeInternal() : currentTimeInternal();
     }
     return toAnimation(m_content.get())->maybeStartAnimationOnCompositor(startTime, timeOffset, m_playbackRate);
 }
 
 void AnimationPlayer::setCompositorPending(bool sourceChanged)
 {
     // FIXME: Animation could notify this directly?
     if (!hasActiveAnimationsOnCompositor()) {
         m_compositorState.release();
     }
@@ skipping 164 matching lines @@
 
 void AnimationPlayer::trace(Visitor* visitor)
 {
     visitor->trace(m_content);
     visitor->trace(m_timeline);
     visitor->trace(m_pendingFinishedEvent);
     EventTargetWithInlineData::trace(visitor);
 }
 
 } // namespace