Chrome OS: Implement Discover UI infrastructure.

This Cl adds basic elements of the Discover App: DiscoverManager,
DiscoverUI, DiscoverModule, etc...

Bug: 828999

Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: I526032df033d68bfae4583e9872262ad8af79219
Reviewed-on: https://chromium-review.googlesource.com/1103457
Commit-Queue: Alexander Alekseev <alemate@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575847}
63 files changed