Currently MockPlatform has shorter lifetime than the isolate that uses
it. This leads to use-after-free races in concurrent tasks that fetch
the mock platform just before it is freed.

This CL ensures that MockPlatform is valid throughout the whole
lifetime of the isolate

Change-Id: Ib94dc7674b9f94833be3372de68209ec38577ca1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2461726
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Emanuel Ziegler <ecmziegler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70407}