GitLab

- Add a CastRunner and main() under //webrunner/app/cast.
- Add "castrunner" executable, package and runner targets to GN.

This rudimentary Runner handles caller requests for cast:// URIs,
extracts the application-Id from them, and if the Id matches a dummy
Id for testing, then it instantiates a component hosting a standard
web URL, for now.

Bug: 893229
Change-Id: Ia2cb5d7c1f69ff0a828af9e5c71b7f4422ce58d3
Reviewed-on: https://chromium-review.googlesource.com/c/1303646
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604071}