Avoid generating invalid options.

static/js/mediastream/peerscreenshare.js

@@ -56,7 +56,10 @@ define(['jquery', 'underscore', 'mediastream/peercall', 'mediastream/tokens'], f
 		// SCTP is supported from Chrome M31.
 		// No need to pass DTLS constraint as it is on by default in Chrome M31.
 		// For SCTP, reliable and ordered is true by default.
-		this.pcConstraints = {};
+		this.pcConstraints = {
+			mandatory: {},
+			optional: []
+		};
 
 		// Inject token into sessiondescription and ice candidate data.
 		this.e.on("sessiondescription icecandidate", _.bind(function(event, data) {

static/js/mediastream/peerxfer.js

@@ -48,11 +48,17 @@ define(['jquery', 'underscore', 'mediastream/peercall', 'mediastream/tokens', 'w
 			audio: false,
 			video: false
 		};
-		this.sdpConstraints = {};
+		this.sdpConstraints = {
+			mandatory: {},
+			optional: []
+		};
 		// SCTP is supported from Chrome M31.
 		// No need to pass DTLS constraint as it is on by default in Chrome M31.
 		// For SCTP, reliable and ordered is true by default.
-		this.pcConstraints = {};
+		this.pcConstraints = {
+			mandatory: {},
+			optional: []
+		};
 
 		// Inject token into sessiondescription and ice candidate data.
 		this.e.on("sessiondescription icecandidate", _.bind(function(event, data) {