[MessageLoop tests] Remove obsolete WITH_TASK_SCHEDULER variant

That variant was useful in the early days of TaskScheduler to ensure
compatibility. It is now in the way as other variants of MessageLoop
are being considered. MessageLoop is independent from TaskScheduler
and the integration is now well-covered implicitly by the vast majority
of tests in the codebase.

R=fdoray@chromium.org

Bug: 708584
Change-Id: Id93d8eb4f970e0aa0e1915ffcbc7471190e3d312
Reviewed-on: https://chromium-review.googlesource.com/1232573
Reviewed-by: Fran├žois Doray <fdoray@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592528}
1 file changed