Slight performance regression by adding locking around feedback
vector get/set. The lock isn't necessary for reads on the main thread,
since the main thread is the only source of change.

Bug: chromium:1144777
Change-Id: I7cc9898ad0d8e8c468ba150c0bc6bef3176fd256
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2516475Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70947}