Bug 1561404 - Ensure thread safety r=jya

Adds a lock in MediaCacheStream::SizeOfExcludingThis to prevent a race
condition with MediaCache::NoteSeek

Differential Revision: https://phabricator.services.mozilla.com/D36880

--HG--
extra : moz-landing-system : lando

dom/media/MediaCache.cpp

@@ -554,6 +554,8 @@ MediaCacheStream::MediaCacheStream(ChannelMediaResource* aClient,
       mIsPrivateBrowsing(aIsPrivateBrowsing) {}
 
 size_t MediaCacheStream::SizeOfExcludingThis(MallocSizeOf aMallocSizeOf) const {
+  AutoLock lock(mMediaCache->Monitor());
+
   // Looks like these are not owned:
   // - mClient
   size_t size = mBlocks.ShallowSizeOfExcludingThis(aMallocSizeOf);