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..788baf7edaeca96850df7393aac5f8ed37513808
--- /dev/null
+++ b/Source/modules/mediasession/HTMLMediaElementMediaSession.idl
@@ -0,0 +1,9 @@
+// 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 {

[philipj_slow, 2015/07/02 20:22:49]

I tested this and verified no change to the generated code:

[
    RuntimeEnabled=MediaSession,
    TypeChecking=Interface,
] partial interface HTMLMediaElement {
    attribute MediaSession? session;
};


Turns out I can't upload changes to this review because "You (philipj@opera.com)
don't own this issue (1217743003)"

+    [RuntimeEnabled=MediaSession] attribute MediaSession? session;
+};