Migrate FrameScheduler-related files to new scheduler directory structure.

Blink scheduler code is transitioning to a new directory structure.
This patch moves a few files related to FrameScheduler, and rewrites
include paths in the affected files.

This patch introduces new directories: platform/scheduler/public and
platform/scheduler/main_thread. DEPS file is added to main_thread/
for moved tests.

Bug: 826203
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I5894ccb0635922957aed6ada2a6e4e2e536d02e7
Reviewed-on: https://chromium-review.googlesource.com/983084
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@{#546756}
40 files changed