GitLab

This is a reland of d4838474

Previously in chromeos/components/sample_system_web_app_ui/BUILD.gn,
the build rule js_library("app") was causing build failures due to a
missing app.js file. This cl fixes the file path for the js files.

Original change's description:
> [system-web-apps] Add chrome-untrusted iframe to sample system web app
>
> An example of how to embed a chrome-untrusted iframe into a system web
> app.
>
> To do so, we need to add abstract methods in WebUI despite WebUIImpl
> being the only class to actually implement it. This is so the
> WebUIController can add requestable schemes so the WebUI can request
> the chrome-untrusted resource.
>
> Bug: 1048951
> Change-Id: Ia3152e21412845cf421df74bb00a16e0a8f264b8
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2037186
> Commit-Queue: Jessica Huang <jessing@google.com>
> Reviewed-by: calamity <calamity@chromium.org>
> Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
> Auto-Submit: Jessica Huang <jessing@google.com>
> Cr-Commit-Position: refs/heads/master@{#742999}

TBR=kinuko@chromium.org

Bug: 1048951
Change-Id: I38307ef0966813426e99e97ecf54403c54124917
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2066404Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743414}