Support Media Session title in demo (#689)

demo/asset_section.js

@@ -181,6 +181,12 @@ shakaDemo.load = function() {
       player.addTextTrack(extraText.uri, extraText.language, extraText.kind,
                           extraText.mime, extraText.codecs);
     });
+
+    // Check if browser supports Media Session first.
+    if ('mediaSession' in navigator) {
+      // Set media session title.
+      navigator.mediaSession.metadata = new MediaMetadata({title: asset.name});
+    }
   }, function(reason) {
     var error = /** @type {!shaka.util.Error} */(reason);
     if (error.code == shaka.util.Error.Code.LOAD_INTERRUPTED) {

externs/mediasession.js

@@ -0,0 +1,39 @@
+/**
+ * @license
+ * Copyright 2017 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+
+/**
+ * @fileoverview Externs for MediaSession based on
+ * {@link https://goo.gl/8QS094 Editor's Draft, 12 January 2017}
+ *
+ * @externs
+ */
+
+
+
+/**
+ * @constructor
+ */
+var MediaMetadata = function(options) {};
+
+
+/** @type {string} */
+MediaMetadata.prototype.title;
+
+
+/** @type {MediaMetadata} */
+Navigator.prototype.mediaSession;