Remove content::kFontListSequenceToken.

content/public/browser/font_list_async.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 CONTENT_PUBLIC_BROWSER_FONT_LIST_ASYNC_H_
 #define CONTENT_PUBLIC_BROWSER_FONT_LIST_ASYNC_H_
 
 #include <memory>
 
-#include "base/callback_forward.h"
+#include "base/callback.h"
 #include "content/common/content_export.h"
 
 namespace base {
 class ListValue;
 }
 
 namespace content {
 
 // Retrieves the list of fonts on the system as a list of strings. It provides
 // a non-blocking interface to GetFontList_SlowBlocking in common/.
 //
 // This function will run asynchronously on a background thread since getting
-// the font list from the system can be slow. This function may be called from
-// any thread that has a BrowserThread::ID. The callback will be executed on
-// the calling thread.
+// the font list from the system can be slow. The callback will be executed on
+// the calling sequence.
 CONTENT_EXPORT void GetFontListAsync(
-    const base::Callback<void(std::unique_ptr<base::ListValue>)>& callback);
+    base::OnceCallback<void(std::unique_ptr<base::ListValue>)> callback);
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_FONT_LIST_ASYNC_H_