[GroupEffect] Remove SingleKeyframeEffectAnimation

SingleKeyframeEffectAnimation was introduced to support multiple effects
per animation for AnimationWorklet. i.e. each animation has a list of
single KeyframeEffects.

However, GroupEffect is now re-designed for WebAnimation. Instead of a
list of KeyframeEffects, an animation will have one AnimationEffect
which could be either a single KeyframeEffect of a GroupEffect which
combines a list of AnimationEffect in different manners. See [1] for
details. Therefore the existing model is obsolete and should be removed.

This patch is just a refactoring and it should not affect any prod
paths.

[1] https://github.com/yi-gu/group_effect

Bug: 767043, 956696
Change-Id: I2b5bc60dea9763ac64e5484e6ba793d8b6404089
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1993429
Commit-Queue: Yi Gu <yigu@chromium.org>
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#730563}

diff --git a/cc/animation/animation_timeline.cc b/cc/animation/animation_timeline.cc
index 0d57f639..143745c 100644
--- a/cc/animation/animation_timeline.cc
+++ b/cc/animation/animation_timeline.cc

@@ -118,7 +118,7 @@
 }
 
 void AnimationTimeline::EraseAnimation(scoped_refptr<Animation> animation) {
-  if (animation->has_element_animations())
+  if (animation->element_animations())
     animation->DetachElement();
   animation->SetAnimationTimeline(nullptr);
   animation->SetAnimationHost(nullptr);