Use PluginPrefs in ChromePluginServiceFilter to check whether a plugin is enabled.

chrome/common/chrome_notification_types.h

 // Copyright (c) 2011 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 CHROME_COMMON_CHROME_NOTIFICATION_TYPES_H_
 #define CHROME_COMMON_CHROME_NOTIFICATION_TYPES_H_
 #pragma once
 
 #include "content/common/content_notification_types.h"
 
@@ skipping 848 matching lines @@
   NOTIFICATION_EDITABLE_ELEMENT_TOUCHED,
 #endif
 
   // Protocol Handler Registry -----------------------------------------------
   // Sent when a ProtocolHandlerRegistry is changed. The source is the profile.
   NOTIFICATION_PROTOCOL_HANDLER_REGISTRY_CHANGED,
 
   // Sent when the cached profile info has changed.
   NOTIFICATION_PROFILE_CACHED_INFO_CHANGED,
 
+  // Sent by the PluginPrefs when there is a change of plugin
+  // enable/disable status.
+  NOTIFICATION_PLUGIN_ENABLE_STATUS_CHANGED,
+
   // Note:-
   // Currently only Content and Chrome define and use notifications.
   // Custom notifications not belonging to Content and Chrome should start
   // from here.
   NOTIFICATION_CHROME_END,
 };
 
 }  // namespace chrome
 
 
 #endif  // CHROME_COMMON_CHROME_NOTIFICATION_TYPES_H_