Bug 1573500 [wpt PR 18399] - Code health refactoring of animation.cc., a=testonly

Automatic update from web-platform-tests
Code health refactoring of animation.cc.

This patch aligns the implementation of web-animations with the spec.
Previously updates were performed via a PlayStateUpdateScope. This
class depended on an extra state (kPending) that is not to spec.
Support for microtasks is also introduced in this patch, which
addresses most of the remaining timing issues with the finished state
of an animation.

Bug: 960944
Change-Id: I62aff2254d2b5265f869415e0f763e140b460c37
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1739386
Reviewed-by: Stephen McGruer <smcgruerchromium.org>
Commit-Queue: Kevin Ellis <keverschromium.org>
Cr-Commit-Position: refs/heads/master{#689018}

--

wpt-commits: 521a012922a378241b46eff13e96bb3c154cb01e
wpt-pr: 18399

UltraBlame original commit: 299f19714e7caa234cc6ae4a2aa737c6ece7dc7e

Author: marco-c

testing/web-platform/tests/web-animations/timing-model/animations/pausing-an-animation.html

@@ -66,7 +66,7 @@
   await animation.ready;
   // If the current time was updated using the new playback rate it will jump
   // back to 25s but if we correctly used the old playback rate the current time
-  // will be >50s.
+  // will be > 50s.
   assert_greater_than(animation.currentTime, 50 * MS_PER_SEC);
 }, 'A pause-pending animation maintains the current time when applying a'
    + ' pending playback rate');