[Chromecast] Fix media session blocking tests.

content/public/test/mock_media_session.cc

Index: content/public/test/mock_media_session.cc
diff --git a/content/public/test/mock_media_session.cc b/content/public/test/mock_media_session.cc
new file mode 100644
index 0000000000000000000000000000000000000000..396d074b21e7abe31690236b850e367f4b47495c
--- /dev/null
+++ b/content/public/test/mock_media_session.cc
@@ -0,0 +1,13 @@
+// Copyright (c) 2017 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.
+
+#include "content/public/test/mock_media_session.h"
+
+namespace content {
+
+MockMediaSession::MockMediaSession() {}
+
+MockMediaSession::~MockMediaSession() {}
+
+}  // namespace content