Added chrome switch that lets tests override URL path, moved GAIA auth extension from CrOS

chrome/browser/ui/webui/chromeos/login/signin_screen_handler.cc

Index: chrome/browser/ui/webui/chromeos/login/signin_screen_handler.cc
diff --git a/chrome/browser/ui/webui/chromeos/login/signin_screen_handler.cc b/chrome/browser/ui/webui/chromeos/login/signin_screen_handler.cc
index 6a42d60c46fa56c070d0f94d4d44d5a8093a3ddf..cb12e5647b3e821ea024c69196c292109ca06c81 100644
--- a/chrome/browser/ui/webui/chromeos/login/signin_screen_handler.cc
+++ b/chrome/browser/ui/webui/chromeos/login/signin_screen_handler.cc
@@ -653,9 +653,13 @@ void SigninScreenHandler::LoadAuthExtension(
   }
 
   params.SetString("gaiaOrigin", GaiaUrls::GetInstance()->gaia_origin_url());
+  const CommandLine* command_line = CommandLine::ForCurrentProcess();
+  if (command_line->HasSwitch(switches::kGaiaUrlPath)) {
+    params.SetString("gaiaUrlPath",
+                     command_line->GetSwitchValueASCII(switches::kGaiaUrlPath));
+  }
 
   // Test automation data:
-  const CommandLine* command_line = CommandLine::ForCurrentProcess();
   if (command_line->HasSwitch(switches::kAuthExtensionPath)) {
     if (!test_user_.empty()) {
       params.SetString("test_email", test_user_);