media: Update comment on SessionExpirationUpdateCB

media/base/content_decryption_module.h

Index: media/base/content_decryption_module.h
diff --git a/media/base/content_decryption_module.h b/media/base/content_decryption_module.h
index e8b36654b614b5685eb737cb4eb0ac1ad01f155b..df06c71207c6cf0233569d66ff51ff3c9189f26d 100644
--- a/media/base/content_decryption_module.h
+++ b/media/base/content_decryption_module.h
@@ -189,6 +189,9 @@ typedef base::Callback<void(const std::string& session_id,
 
 // 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;