Coarsen timestamp for rAF callback and web animations

The timestamps exposed by WAAPI and the requestAnimationFrame
callback should be coarsened if the page is cross-origin-isolated.

Using the same coarsening logic in both to ensure that the numbers
are exactly equal.

This has recently been merged into the HTML spec,
and matches Gecko/WebKit.

Bug: 1422926
Change-Id: I750012854d569ad86174bd4711670a2c6569a020
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4952594
Reviewed-by: David Bokan <bokan@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Noam Rosenthal <nrosenthal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1214838}