Now the mock platform delays all background tasks and forwards them
to the real platform in its destructor.

This fixes a race that happens when the background tasks calls
TestPlatform::MonotonicallyIncreasingTime() while the mock platform
is being destroyed.

BUG: v8:7494
Change-Id: I659ccc19121144152f447d59ff3c5e7ef1bec6d5
Reviewed-on: https://chromium-review.googlesource.com/949202Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51741}