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

content/browser/plugin_service_filter.h

Index: content/browser/plugin_service_filter.h
diff --git a/content/browser/plugin_service_filter.h b/content/browser/plugin_service_filter.h
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+// 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 CONTENT_BROWSER_PLUGIN_FILTER_H_
+#define CONTENT_BROWSER_PLUGIN_FILTER_H_
+#pragma once
+
+class GURL;
+
+namespace webkit {
+namespace npapi {
+struct WebPluginInfo;
+}
+}
+
+namespace content {
+
+class ResourceContext;
+
+// Callback class to let the client filter the list of all installed plug-ins.
+// This class is called on the FILE thread.
+class PluginServiceFilter {
+ public:
+  virtual ~PluginServiceFilter() {}
+  // Whether to use |plugin|. The client can return false to disallow the
+  // plugin, or return true and optionally change the passed in plugin.
+  virtual bool ShouldUsePlugin(
+      int render_process_id,
+      int render_view_id,
+      const ResourceContext& resource_context,
+      const GURL& url,
+      const GURL& policy_url,
+      webkit::npapi::WebPluginInfo* plugin) const = 0;
+};
+
+}  // namespace content
+
+#endif  // CONTENT_BROWSER_PLUGIN_FILTER_H_