GitLab

- Create WebUI
- Add PostMessageAPI* and AddSupervisionAPI* JS classes for communication
with web-content.
- Added switch to control the feature:  ChromeOSAddSupervision


Bug: 937995
Change-Id: I1e65b1a899776b5aa9028921d028030d147baaec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1500832
Commit-Queue: Danan S <danan@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Roger Tawa <rogerta@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659120}