Pass the Isolate/LocalIsolate through to StringTable matchers and
WriteToFlat, so avoid having to get the Isolate via the String, and to
avoid locking on the main thread entirely. This allows us to remove the
String overload of the SharedStringAccessGuardIfNeeded constructor
entirely, to avoid this anti-pattern in the future.

Bug: chromium:1146972
Change-Id: I53bba126b105e1c9629d6e64d8bb574e62e3ad45
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2557988
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71398}