Add missing externs.js files

externs/mediaelements.js

@@ -0,0 +1,29 @@
+/**
+ * @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 HTML Media Elements.
+ *
+ * @externs
+ */
+
+
+/**
+ * @override
+ */
+HTMLMediaElement.prototype.play = function() {};

externs/mediasession.js

@@ -0,0 +1,38 @@
+/**
+ * @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/8QS094i Editor's Draft, 12 January 2017}
+ *
+ * @externs
+ */
+
+
+
+/**
+ * @constructor
+ */
+var MediaMetadata = function() {};
+
+
+
+/**
+ * @constructor
+ */
+Navigator.prototype.mediaSession = function() {};