Bug 1395802. P2 - assert ChannelMediaResource::GetDownloadRate() runs on the main thread. r=cpearce

The only caller is ChannelMediaDecoder::GetStatistics() which runs on the main thread.

MozReview-Commit-ID: CYg3Z3rmlHd

--HG--
extra : rebase_source : c3bf0083522256a6c4f3e83bc817ee7d0ce398bf
extra : intermediate-source : 12bad420f0f3eb9a3993fada2f919d61f60ad392
extra : source : 6044445039445631bbb6cd66607b6f7f72547f18

dom/media/MediaResource.cpp

@@ -1005,6 +1005,7 @@ ChannelMediaResource::Unpin()
 double
 ChannelMediaResource::GetDownloadRate(bool* aIsReliable)
 {
+  MOZ_ASSERT(NS_IsMainThread());
   MutexAutoLock lock(mLock);
   return mChannelStatistics.GetRate(aIsReliable);
 }