Bug 1531505 - Part 1: Ignore source-level msid if media-level msid is present. r=mjf

Differential Revision: https://phabricator.services.mozilla.com/D23202

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

media/webrtc/signaling/src/sdp/SdpHelper.cpp

@@ -400,9 +400,10 @@ nsresult SdpHelper::GetMsids(const SdpMediaSection& msection,
                              std::vector<SdpMsidAttributeList::Msid>* msids) {
   if (msection.GetAttributeList().HasAttribute(SdpAttribute::kMsidAttribute)) {
     *msids = msection.GetAttributeList().GetMsid().mMsids;
+    return NS_OK;
   }
 
-  // Can we find some additional msids in ssrc attributes?
+  // If there are no a=msid, can we find msids in ssrc attributes?
   // (Chrome does not put plain-old msid attributes in its SDP)
   if (msection.GetAttributeList().HasAttribute(SdpAttribute::kSsrcAttribute)) {
     auto& ssrcs = msection.GetAttributeList().GetSsrc().mSsrcs;