--- a/dom/media/PeerConnection.js
+++ b/dom/media/PeerConnection.js
@@ -1039,34 +1039,24 @@ RTCPeerConnection.prototype = {
     throw new this._win.DOMException("getStreamById not yet implemented",
                                      "NotSupportedError");
   },
 
   addTrack: function(track, stream) {
     if (stream.currentTime === undefined) {
       throw new this._win.DOMException("invalid stream.", "InvalidParameterError");
     }
-    if (stream.getTracks().indexOf(track) < 0) {
-      throw new this._win.DOMException("track is not in stream.",
-                                       "InvalidParameterError");
-    }
     this._checkClosed();
     this._senders.forEach(sender => {
       if (sender.track == track) {
         throw new this._win.DOMException("already added.",
                                          "InvalidParameterError");
       }
     });
-    try {
-      this._impl.addTrack(track, stream);
-    } catch (e if (e.result == Cr.NS_ERROR_NOT_IMPLEMENTED)) {
-      throw new this._win.DOMException(
-          "track in constructed stream not yet supported (see Bug 1259236).",
-          "NotSupportedError");
-    }
+    this._impl.addTrack(track, stream);
     let sender = this._win.RTCRtpSender._create(this._win,
                                                 new RTCRtpSender(this, track,
                                                                  stream));
     this._senders.push(sender);
     return sender;
   },
 
   removeTrack: function(sender) {

--- a/media/webrtc/signaling/src/peerconnection/PeerConnectionImpl.cpp
+++ b/media/webrtc/signaling/src/peerconnection/PeerConnectionImpl.cpp
@@ -2259,25 +2259,16 @@ PeerConnectionImpl::AddTrack(MediaStream
 
   return AddTrack(aTrack, aStreams[0]);
 }
 
 nsresult
 PeerConnectionImpl::AddTrack(MediaStreamTrack& aTrack,
                              DOMMediaStream& aMediaStream)
 {
-  if (!aMediaStream.HasTrack(aTrack)) {
-    CSFLogError(logTag, "%s: Track is not in stream", __FUNCTION__);
-    return NS_ERROR_FAILURE;
-  }
-  if (!aMediaStream.OwnsTrack(aTrack)) {
-    CSFLogError(logTag, "%s: Track is not in owned stream (Bug 1259236)", __FUNCTION__);
-    return NS_ERROR_NOT_IMPLEMENTED;
-  }
-
   std::string streamId = PeerConnectionImpl::GetStreamId(aMediaStream);
   std::string trackId = PeerConnectionImpl::GetTrackId(aTrack);
   nsresult res = mMedia->AddTrack(aMediaStream, streamId, aTrack, trackId);
   if (NS_FAILED(res)) {
     return res;
   }
 
   CSFLogDebug(logTag, "Added track (%s) to stream %s",