SetTestSampleTime is called from tests via the advanceTimeAndRefresh API
on DOMWindowUtils, and the expectation is that after it is done, the
time has been advanced and a composite has happened. So we need to
ensure that is the case with WebRender as well. This fixes the issue I
was seeing with test_group_hittest.html and makes it pass.

MozReview-Commit-ID: 86l9mTTwD2v

--HG--
extra : rebase_source : d2921fb0f9b09f4366fb516e4a6254a7f13f3e4e