Fix missing supervised user delete message on CrOS

This was broken in http://crrev.com/1248613003, which replaced JS code to show/hide the message with CSS selectors based on the 'legacy-supervised' class.
However, that class was only used on desktop, not on CrOS.

This CL is a minimal fix, for merging to M48: Set this particular class also on CrOS.
Longer-term, it would be nice to clean up the user pod stuff to be less different on CrOS vs desktop...

BUG=546183

Review URL: https://codereview.chromium.org/1446193002

Cr-Commit-Position: refs/heads/master@{#360037}

ui/login/account_picker/user_pod_row.js

@@ -1028,6 +1028,7 @@ cr.define('login', function() {
         this.setUserPodIconType('child');
       } else if (this.user_.legacySupervisedUser && !this.user_.isDesktopUser) {
         this.setUserPodIconType('legacySupervised');
+        this.classList.add('legacy-supervised');
       } else if (this.multiProfilesPolicyApplied) {
         // Mark user pod as not focusable which in addition to the grayed out
         // filter makes it look in disabled state.