Convert message/title to strings in render process

lib/renderer/window-setup.js

@@ -121,11 +121,11 @@ module.exports = (ipcRenderer, guestInstanceId, openerId, hiddenPage) => {
   }
 
   window.alert = function (message, title) {
-    ipcRenderer.sendSync('ELECTRON_BROWSER_WINDOW_ALERT', message, title)
+    ipcRenderer.sendSync('ELECTRON_BROWSER_WINDOW_ALERT', `${message}`, `${title}`)
   }
 
   window.confirm = function (message, title) {
-    return ipcRenderer.sendSync('ELECTRON_BROWSER_WINDOW_CONFIRM', message, title)
+    return ipcRenderer.sendSync('ELECTRON_BROWSER_WINDOW_CONFIRM', `${message}`, `${title}`)
   }
 
   // But we do not support prompt().