Rename RendererScheduler to WebMainThreadScheduler.

Scheduler classes are being renamed as part of scheduler
architecture refactoring.

RendererScheduler is used in very many places, so this patch affects
a lot of files. The diffs are basically simple renames of classes,
functions or variables.

TBR=kinuko@chromium.org,dalecurtis@chromium.org

Bug: 826203
Change-Id: I4fd701b5e8b35e9ec5ab44f136af2fbe00f07d83
Reviewed-on: https://chromium-review.googlesource.com/994854
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@{#548385}
53 files changed