[Chromedriver] Chromedriver should handle unexpected alert automatically.

chrome/test/chromedriver/chrome/stub_chrome.cc

Index: chrome/test/chromedriver/chrome/stub_chrome.cc
diff --git a/chrome/test/chromedriver/chrome/stub_chrome.cc b/chrome/test/chromedriver/chrome/stub_chrome.cc
index cdf641f8f8fb2e40efd6a75f5c5df5813abb1c34..b0cbd6b9ccb0d484f2a36deb1251dca3de8fda6a 100644
--- a/chrome/test/chromedriver/chrome/stub_chrome.cc
+++ b/chrome/test/chromedriver/chrome/stub_chrome.cc
@@ -58,6 +58,10 @@ std::string StubChrome::page_load_strategy() const {
   return std::string();
 }
 
+std::string StubChrome::UnexpectedAlertBehaviour() const {
+  return std::string();
+}
+
 Status StubChrome::Quit() {
   return Status(kOk);
 }