Bug 1426056. P3 - assert MediaCacheStream::GetLength() is called off the main thread. r=bechen,gerald

MozReview-Commit-ID: 7L8C4f2Lg3C

--HG--
extra : rebase_source : bfa198778bab4b7e0f39d52b292754c2feeff5ec
extra : intermediate-source : 075391df39fd04af006dab86937b37fe3eec80b9
extra : source : ab007303c8bfe447b6a6128adfc55337127923a2

dom/media/MediaCache.cpp

@@ -2423,7 +2423,7 @@ MediaCacheStream::Unpin()
 int64_t
 MediaCacheStream::GetLength()
 {
-  // TODO: Assert non-main thread.
+  MOZ_ASSERT(!NS_IsMainThread());
   AutoLock lock(mMediaCache->Monitor());
   return mStreamLength;
 }