Merge RendererWebSchedulerImpl into MainThreadSchedulerImpl.

This patch moves virtual function implementation of Renderer-
WebSchedulerImpl to MainThreadSchedulerImpl, and remove Renderer-
WebSchedulerImpl. MainThreadSchedulerImpl now inherits from both
WebMainThreadScheduler and ThreadScheduler, just like NonMainThread-
Scheduler.

WebMainThreadScheduler and ThreadScheduler have a substantial number of
overlapping functions. They are implemented in MainThreadSchedulerImpl
at the same time, as those functions are compatible with each other's.
A future cleanup is planned to reduce the overlaps for a cleaner code
structure.

Bug: 826203
Change-Id: Ic32a1175b07a9f9e86123f4a70329b269758a841
Reviewed-on: https://chromium-review.googlesource.com/1058990
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559452}
15 files changed