Add stub for assigning media session to media elements

Source/modules/mediasession/HTMLMediaElementMediaSession.idl

Index: Source/modules/mediasession/HTMLMediaElementMediaSession.idl
diff --git a/Source/modules/mediasession/HTMLMediaElementMediaSession.idl b/Source/modules/mediasession/HTMLMediaElementMediaSession.idl
new file mode 100644
index 0000000000000000000000000000000000000000..91ee9dfe37dc12813e2d3af6a38bc50ebf0fe7c9
--- /dev/null
+++ b/Source/modules/mediasession/HTMLMediaElementMediaSession.idl
@@ -0,0 +1,8 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// https://mediasession.spec.whatwg.org/#extensions-to-the-htmlmediaelement-interface
+[TypeChecking=Interface] partial interface HTMLMediaElement {
+    [RuntimeEnabled=MediaSession] attribute MediaSession? session;
+};