Resetting the flag in the scope desctructor causes concurrent access to
the flag value, triggering a TSAN failure when
--stress-concurrent-inlining is on. The behaviour of the stress flag
was changed in crrev.com/c/2824440 to improve test coverage, so this
CL replaces the scope usage with simple flag assignment.

Bug: v8:11740
Change-Id: Ie706edaa50486dd3b6eaabc9d1f26a8d03068418
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2882801Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74461}