Bug 1785162 - [2/3] refactor to specify stall count and time r=glandium,gsvelto

Refactor to allow specifying a potentially variable stall-count and
-time, rather than a binary stall/don't-stall cue.

No functional changes; this facility will actually be used in the next
commit.

Differential Revision: https://phabricator.services.mozilla.com/D157139