[Animation Worklet] Support effects with no target

Keyframe effect with no target element is allowed in WebAnimation spec
[1]. This patch adds the support and makes sure that adding or removing
the target while the animation is running doesn't affect the result.

[1] https://drafts.csswg.org/web-animations/#dom-keyframeeffect-target

Bug: 781816
Change-Id: I23c5c005e61bb988c51aba60d07fe3f2589dbff1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1572260
Commit-Queue: Yi Gu <yigu@chromium.org>
Reviewed-by: Majid Valipour <majidvp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654167}
4 files changed