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 <[email protected]>
Reviewed-by: Alexander Timin <[email protected]>
Reviewed-by: Gabriel Charette <[email protected]>
Reviewed-by: Chirantan Ekbote <[email protected]>
Cr-Commit-Position: refs/heads/master@{#507961}
18 files changed