- Adds a --harmony-atomics feature to gate Atomics. This allows us to
  independently add SharedArrayBuffer and Atomics on the global object
  of Contexts, which is necessary for migration to a COOP+COEP site
  isolation requirement.

Bug: chromium:923807
Change-Id: If80c12eb86dc0251a5e5fad62a6dd5ced3380b5b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2340322
Commit-Queue: Bill Budge <bbudge@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Ben Smith <binji@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70129}