Add synchronized accessors for ScriptContextTable on NativeContext.
Add corresponding cctest.

Bug: v8:7790
Change-Id: If390f3d4a72441a8b4323e9413d7627cd15514c0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2299372
Commit-Queue: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68981}