add helper to extract stream id from the video element

modules/RTC/RTCUIHelper.js

 /* global $ */
 var RTCBrowserType = require("./RTCBrowserType");
+var RTC = require('./RTC');
 
 var RTCUIHelper = {
 
         if (!RTCBrowserType.isIExplorer()) {
             streamElement.autoplay = true;
         }
+    },
+
+    /**
+     * Extract video stream id from the video element.
+     * @param {Element} element
+     * @returns {string} video stream id or empty string
+     */
+    getVideoId: function (element) {
+        var src = RTC.getVideoSrc(element);
+        if (!src) {
+            return "";
+        }
+
+        if (RTCBrowserType.isFirefox()) {
+            return src.id;
+        }
+
+        return src;
     }
 };
 
-module.exports = RTCUIHelper;
+module.exports = RTCUIHelper;