Use ScopedTaskEnvironment instead of MessageLoop in components unit tests.

ScopedTaskEnvironment allows usage of ThreadTaskRunnerHandle and
base/task_scheduler/post_task.h within its scope. It should be
instantiated in every unit test 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
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Review-Url: https://codereview.chromium.org/2798963003
Cr-Commit-Position: refs/heads/master@{#462510}
27 files changed