With concurrent allocation background threads invoke Log::IsEnabled()
as well. Fix data race here by making is_enabled_ atomic, such that
IsEnabled() remains cheap.

After locking the mutex in MessageBuilder, IsEnabled() needs to be
checked again in case an old value was read. Otherwise we might log
even though logging was already disabled on another thread.

The other direction where a log message isn't logged is deemed
acceptable.

Bug: v8:10315
Change-Id: I32c9dd2e9879fbdb4ca94e080a16ddd875de7c30
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2362948
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69495}