fix(ie11): do not type check for RTCSessionDescription

Error handling was added to ensure some munging helpers
were receiving RTCSessionDescriptions. Temasys is doing
something else and is passing its own object into
the mungers. So, let temasys do that.

modules/RTC/LocalSdpMunger.js

      * @returns {RTCSessionDescription}
      */
     maybeAddMutedLocalVideoTracksToSDP(desc) {
-        if (!desc || !(desc instanceof RTCSessionDescription)) {
-            throw new Error('Incorrect type, expected RTCSessionDescription');
+        if (!desc) {
+            throw new Error('No local description passed in.');
         }
 
         const transformer = new SdpTransformWrap(desc.sdp);

modules/RTC/TraceablePeerConnection.js

  * @param {Object} localDescription the SDP object as defined by WebRTC.
  */
 const enforceSendRecv = function(localDescription) {
-    if (!localDescription
-        || !(localDescription instanceof RTCSessionDescription)) {
-        throw new Error('Incorrect type, expected RTCSessionDescription');
+    if (!localDescription) {
+        throw new Error('No local description passed in.');
     }
 
     const transformer = new SdpTransformWrap(localDescription.sdp);