Remove the usage of BrowserThread::FILE in web_ui_user_script_loader.cc

extensions/browser/user_script_loader.h

Index: extensions/browser/user_script_loader.h
diff --git a/extensions/browser/user_script_loader.h b/extensions/browser/user_script_loader.h
index a6a5d7c9ef23d0845026a8fa4cf6cf0f80ec6517..a24eb01a6e010a106636cd0031330490591b0e76 100644
--- a/extensions/browser/user_script_loader.h
+++ b/extensions/browser/user_script_loader.h
@@ -42,8 +42,8 @@ namespace extensions {
 class UserScriptLoader : public content::NotificationObserver {
  public:
   using LoadScriptsCallback =
-      base::Callback<void(std::unique_ptr<UserScriptList>,
-                          std::unique_ptr<base::SharedMemory>)>;
+      base::OnceCallback<void(std::unique_ptr<UserScriptList>,
+                              std::unique_ptr<base::SharedMemory>)>;
   class Observer {
    public:
     virtual void OnScriptsLoaded(UserScriptLoader* loader) = 0;