[task scheduler] Add support for jobs which only trigger on demand
[task scheduler] Add support for jobs which only trigger on demand

eg. only on tryjobs or force triggers.

Bug: skia:
Change-Id: I924bfa71867437d884fcbce29c239388bcbaf784
Reviewed-on: https://skia-review.googlesource.com/123625
Commit-Queue: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>
Reviewed-by: Ben Wagner <3ef7217be91069877d94f7907ce5479000772cd3@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