Rewrite ComputedAccessibleNodePromiseResolver to use AnimationFrame instead of IdleTask

Idle tasks are too difficult to use in layout tests, and are also not implemented anywhere other than Chrome.

Bug: 792783
Change-Id: I802c2a9cdd292138ff68fefd3f69b71e0f6e2e98
Reviewed-on: https://chromium-review.googlesource.com/923469
Commit-Queue: Alice Boxhall <aboxhall@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537705}
13 files changed