GitLab

Also add WPT that check
- calling updateUI works
- calling updateUI more than once throws an exception

Change-Id: I4b8e1847a09fd84a087d4f4af996ccf19c9f212b
Reviewed-on: https://chromium-review.googlesource.com/1159073
Commit-Queue: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582140}