[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 Therefore the existing model is obsolete and should be removed.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}

• cc/animation/BUILD.gn[diff]
• cc/animation/README.md[diff]
• cc/animation/animation.cc[diff]
• cc/animation/animation.h[diff]
• cc/animation/animation_events.cc[diff]
• cc/animation/animation_events.h[diff]
• cc/animation/animation_host.cc[diff]
• cc/animation/animation_host.h[diff]
• cc/animation/animation_host_perftest.cc[diff]
• cc/animation/animation_timeline.cc[diff]
• cc/animation/animation_timeline_unittest.cc[diff]
• cc/animation/animation_unittest.cc[diff]
• cc/animation/element_animations_unittest.cc[diff]
• cc/animation/keyframe_effect.cc[diff]
• cc/animation/keyframe_effect.h[diff]
• cc/animation/scroll_offset_animations_impl.cc[diff]
• cc/animation/scroll_offset_animations_impl.h[diff]
• cc/animation/single_keyframe_effect_animation.cc[Deleted - diff]
• cc/animation/single_keyframe_effect_animation.h[Deleted - diff]
• cc/animation/worklet_animation.cc[diff]
• cc/animation/worklet_animation.h[diff]
• cc/animation/worklet_animation_unittest.cc[diff]
• cc/test/animation_test_common.cc[diff]
• cc/test/animation_test_common.h[diff]
• cc/test/animation_timelines_test_common.cc[diff]
• cc/test/animation_timelines_test_common.h[diff]
• cc/test/layer_tree_test.cc[diff]
• cc/test/layer_tree_test.h[diff]
• cc/trees/draw_properties_unittest.cc[diff]
• cc/trees/layer_tree_host_unittest_animation.cc[diff]
• third_party/blink/renderer/platform/animation/compositor_animation.cc[diff]
• third_party/blink/renderer/platform/animation/compositor_animation.h[diff]
• third_party/blink/renderer/platform/animation/compositor_animation_test.cc[diff]
• ui/compositor/layer_animator.cc[diff]
• ui/compositor/layer_animator.h[diff]
• ui/compositor/layer_owner_unittest.cc[diff]
• ui/compositor/layer_unittest.cc[diff]