Rename WebThreadImplForRendererScheduler to MainThread.

This is part of ongoing refactoring on WebThread classes.

The new name is... so concise!

Bug: 826203
Change-Id: I3352f7688b7c7e689463dbac8debdd8c3c8c604c
Reviewed-on: https://chromium-review.googlesource.com/c/1295631
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601950}
6 files changed