GitLab

Add a common interface for all tests, so that current tests can work
with multiple SequenceManager backends (MockTaskRunner, MessagePump or
MessageLoop)

Change-Id: I6b37fe16483acf76102fa1b6b0882b9f91a0846a
Reviewed-on: https://chromium-review.googlesource.com/c/1448177
Commit-Queue: Carlos Caballero <carlscab@google.com>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628375}