media: Update comment on SessionExpirationUpdateCB

media/base/content_decryption_module.h

 // 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.
 
 #ifndef MEDIA_BASE_CONTENT_DECRYPTION_MODULE_H_
 #define MEDIA_BASE_CONTENT_DECRYPTION_MODULE_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 171 matching lines @@
 
 // Called when there has been a change in the keys in the session or their
 // status. See http://w3c.github.io/encrypted-media/#dom-evt-keystatuseschange
 typedef base::Callback<void(const std::string& session_id,
                             bool has_additional_usable_key,
                             CdmKeysInfo keys_info)>
     SessionKeysChangeCB;
 
 // Called when the CDM changes the expiration time of a session.
 // See http://w3c.github.io/encrypted-media/#update-expiration
+// A null base::Time() will be translated to NaN in Javascript, which means "no
+// such time exists or if the license explicitly never expires, as determined
+// by the CDM", according to the EME spec.
 typedef base::Callback<void(const std::string& session_id,
                             base::Time new_expiry_time)>
     SessionExpirationUpdateCB;
 
 }  // namespace media
 
 #endif  // MEDIA_BASE_CONTENT_DECRYPTION_MODULE_H_