GitLab

- Change model to be a Polymer UI element
- Leverage existing classes to store data
- Add behavior for setting/getting settings values

Bug: 773928
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I30b5aa71363c0bc2820670e8a9f3fb77b5975477
Reviewed-on: https://chromium-review.googlesource.com/783692Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519877}