GitLab

This simplifies base::MockTimer implementation using overridden task
runner. Also, this moves base::MockTimer to //base/test:test_support.

Tbr: khorimoto@chromium.org, chirantan@chromium.org, sky@chromium.org
Bug: 850247
Change-Id: Id233bfdff24fc258d45c5fb3e6ebe292fc973775
Reviewed-on: https://chromium-review.googlesource.com/1094852
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568356}