MD User Manager: fix tab handling for locked user.

When a locked supervising user has no local credentials, the password
input will be hidden. However, the code currently still tries to focus
that input as the mainInput of the user pod, which puts tab navigation
in a limbo state.

This CL adds a check in DesktopUserPod to focus the correct element
instead of the hidden input while tabbing, when there's no local creds.

Bug: 617062
Change-Id: Ia4e6edcecf8e25813debcdd89dcf72186f04be76
Reviewed-on: https://chromium-review.googlesource.com/741749Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Scott Chen <scottchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512468}

ui/login/account_picker/user_pod_row.js

@@ -2469,7 +2469,7 @@ cr.define('login', function() {
 
     /** @override */
     get mainInput() {
-      if (this.user.needsSignin)
+      if (this.user.needsSignin && this.user.hasLocalCreds)
         return this.passwordElement;
       else
         return this.nameElement;