Implement search answer "dark run" mode.

ui/app_list/app_list_features.cc

 // 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.
 
 #include "ui/app_list/app_list_features.h"
 
 #include "base/feature_list.h"
 #include "base/metrics/field_trial_params.h"
 
 namespace app_list {
 namespace features {
 
 const base::Feature kEnableAnswerCard{"EnableAnswerCard",
                                       base::FEATURE_DISABLED_BY_DEFAULT};
+const base::Feature kEnableAnswerCardDarkRun{"EnableAnswerCardDarkRun",
+                                             base::FEATURE_DISABLED_BY_DEFAULT};
 const base::Feature kEnableFullscreenAppList{"EnableFullscreenAppList",
                                              base::FEATURE_DISABLED_BY_DEFAULT};
 
 bool IsAnswerCardEnabled() {
   static const bool enabled = base::FeatureList::IsEnabled(kEnableAnswerCard);
   return enabled;
 }
 
+bool IsAnswerCardDarkRunEnabled() {
+  static const bool enabled =
+      base::FeatureList::IsEnabled(kEnableAnswerCardDarkRun);
+  return enabled;
+}
+
 bool IsFullscreenAppListEnabled() {
   // Not using local static variable to allow tests to change this value.
   return base::FeatureList::IsEnabled(kEnableFullscreenAppList);
 }
 
 std::string AnswerServerUrl() {
   return base::GetFieldTrialParamValueByFeature(kEnableAnswerCard, "ServerUrl");
 }
 
 }  // namespace features
 }  // namespace app_list