media: Make MediaKeys ref-counted.

media/base/media_keys.cc

 // Copyright 2013 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 "media/base/media_keys.h"
 
 namespace media {
 
 MediaKeys::MediaKeys() {}
 
 MediaKeys::~MediaKeys() {}
 
+void MediaKeys::DeleteOnCorrectThread() const {
+  delete this;
+}
+
+// static
+void MediaKeysTraits::Destruct(const MediaKeys* media_keys) {
+  media_keys->DeleteOnCorrectThread();
+}
+
 }  // namespace media