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

chrome/browser/plugin_prefs.cc

 // 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.
 
 #include "chrome/browser/plugin_prefs.h"
 
 #include <string>
 
 #include "base/command_line.h"
 #include "base/memory/scoped_ptr.h"
@@ skipping 389 matching lines @@
     return;
   notify_pending_ = true;
   MessageLoop::current()->PostTask(
       FROM_HERE,
       NewRunnableMethod(this, &PluginPrefs::OnNotifyPluginStatusChanged));
 }
 
 void PluginPrefs::OnNotifyPluginStatusChanged() {
   notify_pending_ = false;
   NotificationService::current()->Notify(
-      content::NOTIFICATION_PLUGIN_ENABLE_STATUS_CHANGED,
+      chrome::NOTIFICATION_PLUGIN_ENABLE_STATUS_CHANGED,
       Source<PluginPrefs>(this),
       NotificationService::NoDetails());
 }
 
 /*static*/
 void PluginPrefs::RegisterPrefs(PrefService* prefs) {
   FilePath internal_dir;
   PathService::Get(chrome::DIR_INTERNAL_PLUGINS, &internal_dir);
   prefs->RegisterFilePathPref(prefs::kPluginsLastInternalDirectory,
                               internal_dir,
                               PrefService::UNSYNCABLE_PREF);
   prefs->RegisterListPref(prefs::kPluginsDisabledPlugins,
                           PrefService::UNSYNCABLE_PREF);
   prefs->RegisterListPref(prefs::kPluginsDisabledPluginsExceptions,
                           PrefService::UNSYNCABLE_PREF);
   prefs->RegisterListPref(prefs::kPluginsEnabledPlugins,
                           PrefService::UNSYNCABLE_PREF);
 }