scheduler: Add enum base::Nestable

This CL introduces a new enum base::Nestable to replace current boolean
values for code health.

Bug: 762453
Change-Id: Ieffcc3fa95337cacefcd390ad3ed90d5d0f4077e
Reviewed-on: https://chromium-review.googlesource.com/701909
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Chirantan Ekbote <chirantan@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#507961}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 64853efbe61349fa76a5403ff9fb448638ffb585
1 file changed