Implement search answer "dark run" mode.

ui/app_list/app_list_features.h

 // Copyright 2017 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 UI_APP_LIST_APP_LIST_FEATURES_H_
 #define UI_APP_LIST_APP_LIST_FEATURES_H_
 
 #include <string>
 
 #include "ui/app_list/app_list_export.h"
 
 namespace base {
 struct Feature;
 }
 
 namespace app_list {
 namespace features {
 
 // Please keep these features sorted.
 
 // Enables the answer card in the app list.
 APP_LIST_EXPORT extern const base::Feature kEnableAnswerCard;
 
+// Enables the dark run answer card in the app list. In this mode, answer cards
+// may be loaded via mock URLs and are not shown to the user.
+APP_LIST_EXPORT extern const base::Feature kEnableAnswerCardDarkRun;
+
 // Enables the fullscreen app list.
 APP_LIST_EXPORT extern const base::Feature kEnableFullscreenAppList;
 
 bool APP_LIST_EXPORT IsAnswerCardEnabled();
+bool APP_LIST_EXPORT IsAnswerCardDarkRunEnabled();
 bool APP_LIST_EXPORT IsFullscreenAppListEnabled();
 std::string APP_LIST_EXPORT AnswerServerUrl();
 
 }  // namespace features
 }  // namespace app_list
 
 #endif  // UI_APP_LIST_APP_LIST_FEATURES_H_