Implement search answer "dark run" mode.

ui/app_list/app_list_features.cc

Index: ui/app_list/app_list_features.cc
diff --git a/ui/app_list/app_list_features.cc b/ui/app_list/app_list_features.cc
index 36cadbf69f325efaad7f51d02f83d96541834083..d00e82a1d6128384451ee3d3aab8f8a3bb6c1ffd 100644
--- a/ui/app_list/app_list_features.cc
+++ b/ui/app_list/app_list_features.cc
@@ -12,6 +12,8 @@ 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};
 
@@ -20,6 +22,12 @@ bool IsAnswerCardEnabled() {
   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);