[Chromedriver] Chromedriver should handle unexpected alert automatically.

chrome/test/chromedriver/capabilities.h

 // Copyright (c) 2013 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 CHROME_TEST_CHROMEDRIVER_CAPABILITIES_H_
 #define CHROME_TEST_CHROMEDRIVER_CAPABILITIES_H_
 
 #include <stddef.h>
 
 #include <map>
@@ skipping 126 matching lines @@
 
   std::string log_path;
 
   LoggingPrefs logging_prefs;
 
   // If set, enable minidump for chrome crashes and save to this directory.
   std::string minidump_path;
 
   std::string page_load_strategy;
 
+  std::string unexpected_alert_behaviour;
+
   bool network_emulation_enabled;
 
   PerfLoggingPrefs perf_logging_prefs;
 
   std::unique_ptr<base::DictionaryValue> prefs;
 
   Switches switches;
 
   std::set<WebViewInfo::Type> window_types;
 };
 
 #endif  // CHROME_TEST_CHROMEDRIVER_CAPABILITIES_H_