It creates a SharedMutexGuard if the boolean parameter in the
constructor is true.

Bug: v8:7790
Change-Id: I77647f648717af05b1239576b690aec6f4e8b268
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2531790
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71162}