Cleanup: Put more plugin code behind the ENABLE_PLUGINS ifdef.

content/common/pepper_plugin_list.h

 // Copyright (c) 2012 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_COMMON_PEPPER_PLUGIN_LIST_H_
 #define CONTENT_COMMON_PEPPER_PLUGIN_LIST_H_
 
 #include <vector>
 
+#if !defined(ENABLE_PLUGINS)
+#error "Plugins should be enabled"
+#endif
+
 namespace content {
+
 struct PepperPluginInfo;
 struct WebPluginInfo;
 
 // Constructs a PepperPluginInfo from a WebPluginInfo. Returns false if
 // the operation is not possible, in particular the WebPluginInfo::type
 // must be one of the pepper types.
 bool MakePepperPluginInfo(const WebPluginInfo& webplugin_info,
                           PepperPluginInfo* pepper_info);
 
 // Computes the list of known pepper plugins.
 void ComputePepperPluginList(std::vector<PepperPluginInfo>* plugins);
 
 }  // namespace content
 
 #endif  // CONTENT_COMMON_PEPPER_PLUGIN_LIST_H_