Convert enable_plugins to a buildflag header.

content/browser/plugin_private_storage_helper.h

 // Copyright 2016 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_PRIVATE_STORAGE_HELPER_H_
 #define CONTENT_BROWSER_PLUGIN_PRIVATE_STORAGE_HELPER_H_
 
-#if !defined(ENABLE_PLUGINS)
+#include "ppapi/features/features.h"
+
+#if !BUILDFLAG(ENABLE_PLUGINS)
 #error This file should only be included when plugins are enabled.
 #endif
 
 #include "base/callback_forward.h"
 #include "base/memory/ref_counted.h"
 #include "base/time/time.h"
 #include "url/gurl.h"
 
 namespace storage {
 class FileSystemContext;
 }
 
 namespace content {
 
 // Clear the plugin private filesystem data in |filesystem_context| for
 // |storage_origin| if any file has a last modified time between |begin|
 // and |end|. If |storage_origin| is not specified, then all available
 // origins are checked. |callback| is called when the operation is complete.
 // This must be called on the file task runner.
 void ClearPluginPrivateDataOnFileTaskRunner(
     scoped_refptr<storage::FileSystemContext> filesystem_context,
     const GURL& storage_origin,
     const base::Time begin,
     const base::Time end,
     const base::Closure& callback);
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_PLUGIN_PRIVATE_STORAGE_HELPER_H_