Change Isolate::code_coverage_mode to an atomic such that access from
the background thread is safe.

Bug: v8:11378
Change-Id: I26d6915b1662ba022ea6a173a87d184d3ac7cd3b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2666691
Commit-Queue: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Auto-Submit: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72530}