Layering: Allow hosts to cleanly specify Promise Job semantics (#1597)

This layering change retains the PendingJob concept but sends
PendingJob records off to the host to be dispatched at the appropriate
time, replacing the RunJobs algorithm. The change is expected to
help specify the interaction between Promises, WeakRefs and the Web
more precisely, while maintaining all current invariants.

Co-authored-by: Domenic Denicola <d@domenic.me>
Co-authored-by: Daniel Ehrenberg <littledan@chromium.org>
Co-authored-by: Shu-yu Guo <syg@chromium.org>