This adds a skeleton SWA that will be used to replace the current
connectivity diagnostics Chrome app with a WebUI based app. This SWA is
gated behind a feature flag to hide it during development.

Bug: chromium:1140611
Change-Id: I52ffd2c21538a26d770b6344d3b6fe4f8a4fec5b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2490504Reviewed-by: Brian White <bcwhite@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Auto-Submit: Trent Begin <tbegin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821857}