If we have a regular isolate (or none at all), we can skip acquiring
the lock check and DCHECK that we are calling from the main thread.
If we have a LocalIsolate, we acquire the string lock if needed.

Bug: v8:7790
Change-Id: Ie3562e8172a3e3eca8d194e8652cb881f765cdb8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2551102
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71340}