Add Platform test helper allowing custom scheduler.

This patch adds a new test utility for Blink platform. It's called
TestingPlatformSupportWithCustomScheduler, and is a natural extension
of TestingPlatformSupport.

The new utility removes the need of writing custom Platform and
WebThread in tests. Essentially, the objective of this CL is to shift
the responsibility of writing custom WebThread from each test to
platform/testing. Two affected tests are rewritten to use it.

Bug: 826203
Change-Id: I41160fe50286adc3acb09dbd128d654c356d4355
Reviewed-on: https://chromium-review.googlesource.com/1170702
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582883}
5 files changed