Revert of Use ScopedTaskEnvironment instead of MessageLoop in chrome unit tests. (patchset #1 id:1 of https://codereview.chromium.org/2797293002/ )

Reason for revert:
Test timeout in MutableProfileOAuth2TokenServiceDelegateTest.ShutdownService

Original issue's description:
> Use ScopedTaskEnvironment instead of MessageLoop in chrome 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
> R=gab@chromium.org
> TBR=sky@chromium.org, vitalybuka@chromium.org
>
> Review-Url: https://codereview.chromium.org/2797293002
> Cr-Commit-Position: refs/heads/master@{#462492}
> Committed: https://chromium.googlesource.com/chromium/src/+/0bc320f205ad411a7ab935fa79879ff75e8a7fc9

TBR=gab@chromium.org,sky@chromium.org,vitalybuka@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=708584

Review-Url: https://codereview.chromium.org/2799743004
Cr-Commit-Position: refs/heads/master@{#462521}
7 files changed