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}