Issue #2332 - Update most tests to skip RTCSessionDescription construction

Backport of https://bugzilla.mozilla.org/show_bug.cgi?id=1263312 part 6

dom/media/tests/mochitest/test_peerConnection_localRollback.html

@@ -23,10 +23,9 @@
         },
 
         function PC_REMOTE_ROLLBACK(test) {
-          return test.setLocalDescription(
-              test.pcRemote,
-              new RTCSessionDescription({ type: "rollback", sdp: ""}),
-              STABLE);
+          return test.setLocalDescription(test.pcRemote,
+                                          { type: "rollback", sdp: "" },
+                                          STABLE);
         },
 
         // Rolling back should shut down gathering