Add empty RTCSessionDescription interface

components/script/dom/webidls/RTCSessionDescription.webidl

@@ -0,0 +1,25 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+// https://w3c.github.io/webrtc-pc/#rtcsessiondescription-class
+
+[Constructor(RTCSessionDescriptionInit descriptionInitDict),
+ Exposed=Window, Pref="dom.webrtc.enabled"]
+interface RTCSessionDescription {
+    // readonly attribute RTCSdpType type;
+    // readonly attribute DOMString  sdp;
+    // [Default] object toJSON();
+};
+
+dictionary RTCSessionDescriptionInit {
+    required RTCSdpType type;
+             DOMString  sdp = "";
+};
+
+enum RTCSdpType {
+    "offer",
+    "pranswer",
+    "answer",
+    "rollback"
+};