Fix reference sample (Dash-Industry-Forum#4843)

* Fix disabling control bar once player stopped

* Fix video element css (aspect ratio)

Author: bbert

samples/dash-if-reference-player/app/css/main.css

@@ -159,6 +159,7 @@ a:hover {
     width: 100%;
     height: auto;
     margin: auto;
+    aspect-ratio: 16 / 9;
 }
 
 .col-md-9 {

samples/dash-if-reference-player/app/main.js

@@ -1205,6 +1205,7 @@ app.controller('DashController', ['$scope', '$window', 'sources', 'contributors'
     };
 
     $scope.doStop = function () {
+        $scope.controlbar.disable();
         $scope.player.attachSource(null);
         $scope.controlbar.reset();
         $scope.conformanceViolations = [];