Currently MockPlatform has shorter lifetime than the isolate that uses
it. Creating isolate before MockPlatform leads to races in concurrent
tasks that were scheduled before the MockPlatform replaced the default
TestPlatform. This caused issues after landing
https://chromium-review.googlesource.com/c/v8/v8/+/2561198

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

Bug: v8:11198
Change-Id: I79f82712165d1f1fbe0fe9af68aedf126e54b241
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2562121Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71443}