scheduler: Rename WebScheduler to ThreadScheduler.

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

The header file is moved to its new location: scheduler/public, since
the file is referred from Blink.

Bug: 826203
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I1c28c5c59ea8a2efd42eb896d89cd7539df4f609
Reviewed-on: https://chromium-review.googlesource.com/1029794
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554311}
70 files changed