Bug 1180403: P1. Ensure buffered index is up to date. r=cpearce

dom/media/fmp4/MP4Demuxer.cpp

@@ -325,6 +325,7 @@ MP4TrackDemuxer::SkipToNextRandomAccessPoint(media::TimeUnit aTimeThreshold)
 int64_t
 MP4TrackDemuxer::GetEvictionOffset(media::TimeUnit aTime)
 {
+  EnsureUpToDateIndex();
   MonitorAutoLock mon(mMonitor);
   uint64_t offset = mIndex->GetEvictionOffset(aTime.ToMicroseconds());
   return int64_t(offset == std::numeric_limits<uint64_t>::max() ? 0 : offset);