Platform::GetForegroundTaskRunner() can only be used after attaching
an Isolate in V8. Work around that problem by getting the runner only
when needed.

Bug: chromium:1056170
Change-Id: If15ec691e7f5cf11be8b7a3bc18827246ac083d6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2674009
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72527}