Make sure we can't get the Isolate for writable off-thread space
objects, to avoid leaking the Isolate into off-thread compilation.

Bug: chromium:1011762
Change-Id: I5c4316e751736b8c8235fdcc8949d52b78313f38
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2043791
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66194}