Revert "Add MOCK_TIME mode to ScopedTaskEnvironment :)"

This reverts commit 5e2df665dfce3a3aceb4852d90ad55d31fc36f20.

Reason for revert: Suspect causing flaky ProfileShortcutManagerTest.
Bug: 787683

Original change's description:
> Add MOCK_TIME mode to ScopedTaskEnvironment :)
> 
> Taking advantage of the new kBoundToThread mode on
> TestMockTimeTaskRunner.
> 
> This change also required tweaking the
> ScopedTaskEnvironment::RunUntilIdle() logic as RunLoop().Run() on
> TestMockTimeTaskRunner results in advancing time when there's no
> request to quit-when-idle which is undesired here. New logic gets rid
> of need for |on_queue_empty_closure_| and I think is simpler overall.
> As of patch set 20, this new RunUntilIdle() logic also avoids using
> TaskScheduler::FlushForTesting() as that can result in hangs should a
> TaskScheduler task synchronously block on the main thread.
> 
> R=​fdoray@chromium.org
> TBR=gab@chromium.org (IWYU fixes)
> 
> Bug: 708584
> Change-Id: I76ba55ec64d398151420379d3fcdcd5186fbceb8
> Reviewed-on: https://chromium-review.googlesource.com/638550
> Commit-Queue: Gabriel Charette <gab@chromium.org>
> Reviewed-by: Gabriel Charette <gab@chromium.org>
> Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
> Reviewed-by: François Doray <fdoray@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#518433}

TBR=khorimoto@chromium.org,gab@chromium.org,fdoray@chromium.org,robliao@chromium.org

Change-Id: I8a8822b221c73159f02297a6b7bdd60ddc5c49bc
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 708584
Reviewed-on: https://chromium-review.googlesource.com/784512
Reviewed-by: Matt Giuca <mgiuca@chromium.org>
Commit-Queue: Matt Giuca <mgiuca@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518560}
8 files changed