The field is immutable after initialization and thus should be set
non-atomically on the main thread, and read non-atomically on the
background thread. But TSAN support for generated code turns all field
accesses into relaxed atomic accesses, leading to this race detection.
Silence it by making the read relaxed as well.

Bug: chromium:1236302,v8:7790
Change-Id: I47979b2dbf61a65a9e92453324fe2b255fafd30d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3070700
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76080}