This assures that MergeableUpdate tasks that lazy push job will actually
have those jobs run instead of being added after the lone callback update
to call JobQueueGroup::pushLazyJobs() already ran.
This also makes it more obvious that push will happen, since a mergeable
update is added each time lazyPush() is called and a job is buffered,
rather than rely on some magic callback enqueued into DeferredUpdates at
just the right point in multiple entry points.
Bug: T207809
Change-Id: I13382ef4a17a9ba0fd3f9964b8c62f564e47e42d
