This CL makes the ScriptContextTable concurrently accessible from the
background thread (in particular ScriptContextTable::get_context).
A cctest is added to check synchronization with tsan.

Bug: v8:7790
Change-Id: I2e2dc8c6a7cfa369787959c4d5ed5f357f4720fa
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2260567Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68979}