While so far this should only happen in tests in test-log.cc, it can
happen that background threads using Logger::is_logging() race with
Logger::TearDownAndGetLogFile().

Fix the race by protecting is_logging_ with the mutex that is also used
for writing log messages. Logger::is_logging_ now becomes relaxed
atomic, such that code for logging isn't required to lock the mutex
to check whether logging is enabled.

Also remove Log::IsEnabled() in favor of Logger::is_logging() to avoid
checking both flags since both are the same.

Bug: v8:10315
Change-Id: Ic14e7f74334eb8a8438abad82ad227d1e6752bb8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2416488
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69973}