Fix XSS in supervised user interstitial

BUG=797525

Change-Id: Ib5cfa732b0f4de8645031c0166e4d67633a65c93
Reviewed-on: https://chromium-review.googlesource.com/844075
Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Commit-Queue: Rob Wu <rob@robwu.nl>
Cr-Commit-Position: refs/heads/master@{#526158}