Implement search answer "dark run" mode.

chrome/browser/ui/app_list/search_answer_web_contents_delegate.h

Index: chrome/browser/ui/app_list/search_answer_web_contents_delegate.h
diff --git a/chrome/browser/ui/app_list/search_answer_web_contents_delegate.h b/chrome/browser/ui/app_list/search_answer_web_contents_delegate.h
index dd65088e7a2a4f39b267ac4532e75282a5aae44a..56fa722b2fd7eff12f5a9912d611dff5850e052b 100644
--- a/chrome/browser/ui/app_list/search_answer_web_contents_delegate.h
+++ b/chrome/browser/ui/app_list/search_answer_web_contents_delegate.h
@@ -94,6 +94,10 @@ class SearchAnswerWebContentsDelegate : public content::WebContentsDelegate,
   // Time when the current server response loaded.
   base::TimeTicks answer_loaded_time_;
 
+  // When in the dark run mode, indicates whether we mimic that the server
+  // response contains an answer.
+  bool dark_run_received_answer_ = false;
+
   DISALLOW_COPY_AND_ASSIGN(SearchAnswerWebContentsDelegate);
 };