The {id_} stored in {ThreadId} should not be atomic. Only getting a new
id for the current thread needs to be atomic. If any user of {ThreadId}
needs atomicity, that user should wrap {ThreadId} in a {std::atomic}
instead.

Drive-by: Remove {Equals} method, use {operator==} instead.
Drive-by: Move static methods after member methods.

R=ishell@chromium.org

Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel
Bug: v8:8834
Change-Id: Id0470eb2fa907948843ac1153e2dc5dcd9a8fbc8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1494006Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60146}