The test creates a mock platform. The bug was that the lifetime of the
mock platform was shoter than the lifetime of the isolate. Even though
the mock platform restores the old platfrom, a background thread may
still have a pointer to the mock platform leading to UAF.

Bug: v8:10690
Tbr: dinfuehr@chromium.rg
Change-Id: Ic14bf408e5e3e9e7d07e01af545bb88c21462300
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2290850Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68777}