Use ScopedTaskEnvironment instead of MessageLoop in tests that use v8.

This is a prerequisite to migrate v8 from base::WorkerPool to
base::TaskScheduler.

ScopedTaskEnvironment allows usage of ThreadTaskRunnerHandle and
base/task_scheduler/post_task.h within its scope. It should be
instantiated in everytest that uses either of these APIs
(i.e. no test should instantiate a MessageLoop directly).

Motivation for ScopedTaskEnvironment can be found in:
https://docs.google.com/document/d/1QabRo8c7D9LsYY3cEcaPQbOCLo8Tu-6VLykYXyl3Pkk/edit

BUG=708584

Review-Url: https://codereview.chromium.org/2852373004
Cr-Commit-Position: refs/heads/master@{#470007}
14 files changed