Currently MockPlatformForUnmapper 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 MockPlatformForUnmapper is valid throughout the whole
lifetime of the isolate

Change-Id: I94a658bf7eb70d924a19522dab09744f21782972
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2502809Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70826}