Made sure empty-call-then-add-one-stream test doesn't do simultaneous negotiations.

content/test/data/media/webrtc_test_utilities.js

Index: content/test/data/media/webrtc_test_utilities.js
diff --git a/content/test/data/media/webrtc_test_utilities.js b/content/test/data/media/webrtc_test_utilities.js
index dae549fe3512fac3fb672c32d49624af2dc9f9b3..61472d19be07fa9e6cf8385f901707daaafb9d5a 100644
--- a/content/test/data/media/webrtc_test_utilities.js
+++ b/content/test/data/media/webrtc_test_utilities.js
@@ -44,6 +44,16 @@ function waitForVideo(videoElement) {
   detectVideoIn(videoElement, function () { eventOccured(); });
 }
 
+function waitForConnectionToStabilize(peerConnection) {
+  addExpectedEvent();
+  var waitForStabilization = setInterval(function() {
+    if (peerConnection.signalingState == 'stable') {
+      clearInterval(waitForStabilization);
+      eventOccured();
+    }
+  }, 100);
+}
+
 function addExpectedEvent() {
   ++gNumberOfExpectedEvents;
 }