We should only assert that the caller is requesting the first frame or
we have advanced to or beyond the expected initial frame, when we
successfully return a frame. This is because FrameAnimator will request
on refresh ticks for the current frame again, until it observes it. If
decoding is still behind, then we likely still have frames to
auto-advance, and we will trip the assert.
Differential Revision: https://phabricator.services.mozilla.com/D9507
d78eab076c