Check username validity in the FakeAuthPolicyClient

chrome/browser/resources/chromeos/login/offline_ad_login.js

Index: chrome/browser/resources/chromeos/login/offline_ad_login.js
diff --git a/chrome/browser/resources/chromeos/login/offline_ad_login.js b/chrome/browser/resources/chromeos/login/offline_ad_login.js
index f722ecf1f85e6f40e10135a3ac3da9a88f99d79c..6c202bc0485dd5a3109fd949f4867a08f34134b6 100644
--- a/chrome/browser/resources/chromeos/login/offline_ad_login.js
+++ b/chrome/browser/resources/chromeos/login/offline_ad_login.js
@@ -124,6 +124,8 @@ Polymer({
   onSubmit_: function() {
     if (this.showMachineInput && !this.$.machineNameInput.checkValidity())
       return;
+    if (!this.$.userInput.checkValidity())
+      return;
     if (!this.$.passwordInput.checkValidity())
       return;
     var user = /** @type {string} */ (this.$.userInput.value);