There is a data race if several background threads check for a tracing
flag concurrently. Both will call {GetCategoryGroupEnabledInternal}.
The first one not find the category in the {g_category_group_enabled}
array, and hence will add it and call {UpdateCategoryGroupEnabledFlag}
to initialize the flag. The second thread then finds the entry in the
array and reads it without any synchronization, which is a data race.

Since we do not really care about this race, we just use a
{Relaxed_Load} to read the field. TSan is fine with that.

R=yangguo@chromium.org
CC=ofrobots@google.com

Bug: v8:8221
Change-Id: Ie09141e3d845956d3c487a463f00b7d6cd413513
Reviewed-on: https://chromium-review.googlesource.com/1245424Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56256}