Web Animations: Compositor timeOffset should be sensitive to playback rate

Source/core/animation/AnimationPlayer.cpp

Index: Source/core/animation/AnimationPlayer.cpp
diff --git a/Source/core/animation/AnimationPlayer.cpp b/Source/core/animation/AnimationPlayer.cpp
index 25e505ad635548f48369f9db34319474f24fd1d4..61807539607ad24917ff29146a367abac146c846 100644
--- a/Source/core/animation/AnimationPlayer.cpp
+++ b/Source/core/animation/AnimationPlayer.cpp
@@ -623,7 +623,7 @@ bool AnimationPlayer::maybeStartAnimationOnCompositor()
     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);
 }