This is a reland of 61d42c94.
Switched to acquire/release memory order.

Original change's description:
> Use std::atomic in Cancelable
>
> Avoid deprecated base::AtomicValue, use std::atomic instead.
> Plus minor drive-by cleanups.
>
> R=mstarzinger@chromium.org
>
> Bug: v8:8238
> Change-Id: I47a1f00e26a843b60380c50399eedc49d859830a
> Reviewed-on: https://chromium-review.googlesource.com/c/1326463
> Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
> Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#57386}

Bug: v8:8238
Change-Id: I992a49d29af68cfa45bfae1bfb8dba5b47f94b25
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_isolates_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/c/1329164
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57394}