We have recursive calls such ThinStrings where we go String::Get into
ThinString::Get into String::Get again for the internalized string. If
we need to, we would acquire the StringAccessGuard in the first
String::Get and it wouldn't be needed to be re-acquired for the second
String::Get. Trying to re-acquire it would in fact be an error since we
are already holding the lock.

The code, however, didn't know if we acquired it or not. It was working
correctly due to the way the methods were defined and called. By passing
down the access guard through the Get() calls we make this interaction
explicit.

Also add some thin string tests to test the interaction.

Bug: v8:7790
Change-Id: I1181edec1e802cb754c4d1d1ac268577257b92f3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2936598
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74984}