Add a requirement to String::GetChars that we either have a string
access lock, or a string access lock is not needed. This prevents us
from reading strings during internalization that may be in the middle
of being made external.

To avoid taking the lock too often when known to be unnecessary (e.g.
for strings that were only just created), there's now a static
SharedStringAccessGuardIfNeeded::NotNeeded(). This is hopefully ugly
enough that it's used sparingly.

One fix required for this is to enter the Isolate when tearing down
IsolateData in inspector tests -- this is so that the V8Inspector
instance being torn down will see the current Isolate and be able to
verify its thread id against the current thread.

Bug: chromium:1011762, chromium:1148680
Change-Id: Ic5d29c1b066ebae5a351c7b4bb116b9b1bf61889
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2536465
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71197}