Add WebUI at chrome://welcome/apps.
WebUI is only shown when NuxGoogleApps feature is enabled.

Bug: 832938
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: I46721ac4474a4335d9c0dfdb977e09622ae0ac1c
Reviewed-on: https://chromium-review.googlesource.com/1066503
Commit-Queue: Hector Carmona <hcarmona@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569424}