[task scheduler] Add TasksCfg.Validate()
[task scheduler] Add TasksCfg.Validate()

BUG=skia:

Change-Id: Iea1339c740c8547ffa87cc5c918c814b26f1b799
Reviewed-on: https://skia-review.googlesource.com/2742
Reviewed-by: Ben Wagner <3ef7217be91069877d94f7907ce5479000772cd3@google.com>
Commit-Queue: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot