[Chromedriver] Chromedriver should handle unexpected alert automatically.

chrome/test/chromedriver/capabilities.h

Index: chrome/test/chromedriver/capabilities.h
diff --git a/chrome/test/chromedriver/capabilities.h b/chrome/test/chromedriver/capabilities.h
index 49a3a72bd82868a0091e84cae586ed0601407047..22ede2b2f4f157a133d9e0f2c6418376da4700f8 100644
--- a/chrome/test/chromedriver/capabilities.h
+++ b/chrome/test/chromedriver/capabilities.h
@@ -144,6 +144,8 @@ struct Capabilities {
 
   std::string page_load_strategy;
 
+  std::string unexpected_alert_behaviour;
+
   bool network_emulation_enabled;
 
   PerfLoggingPrefs perf_logging_prefs;