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/+/2502808

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

Change-Id: Ia888fba93819ea98b935a1d36307d98dd358fcad
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2507379Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70950}