Fix bug with login-prompt-visible not being sent in some rare cases.

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 6a7833ec5f780ba6527b23e474ace9328104747e..cbdcc8ac3e62f6970a1d48344fc4c2bcd538280f 100644
--- a/chrome/browser/ui/webui/chromeos/login/signin_screen_handler.cc
+++ b/chrome/browser/ui/webui/chromeos/login/signin_screen_handler.cc
@@ -1009,6 +1009,7 @@ void SigninScreenHandler::SendUserList(bool animated) {
 
 void SigninScreenHandler::HandleAccountPickerReady(
     const base::ListValue* args) {
+  LOG(INFO) << "Login WebUI >> AccountPickerReady";
 
   PrefService* prefs = g_browser_process->local_state();
   if (prefs->GetBoolean(prefs::kFactoryResetRequested)) {
@@ -1139,8 +1140,14 @@ void SigninScreenHandler::HandleOpenProxySettings(const base::ListValue* args) {
 }
 
 void SigninScreenHandler::HandleLoginVisible(const base::ListValue* args) {
-  LOG(INFO) << "Login WebUI >> LoginVisible, webui_visible_: "
-            << webui_visible_;
+  std::string source;
+  if (!args->GetString(0, &source)) {
+    NOTREACHED();
+    return;
+  }
+
+  LOG(INFO) << "Login WebUI >> LoginVisible, source: " << source
+            << ", webui_visible_: " << webui_visible_;
   if (!webui_visible_) {
     // There might be multiple messages from OOBE UI so send notifications after
     // the first one only.