Bug
1004871 part 7 - Remove event queueing from GetPositionInIteration and do it in GetEventsAt; r=dholbert
This patch simply shifts the event-related code from GetPositionInIteration to
GetEventsAt. Although there are simplifications that could be done to
GetEventsAt, they are deferred to a subsequent patch so as not to obscure the
translation of code from one function to another.
As a result of moving event-related handling from GetPositionInIteration it no
longer needs to support different main-thread vs compositor modes.