Chromium supports communication with media devices on the local network via
the Cast and DIAL protocols. This takes two primary forms:
1. Messaging, where strings representing state information are passed between
the client and a dedicated receiver app on the media device. The receiver
app communicates directly with an app-specific backend service to retrieve
and possibly control media playback.
2. Tab/desktop mirroring, where the media contents are streamed directly from
the browser to a generic streaming app on the media device and playback is
controlled by the browser.
This change adds support for device discovery and messaging (but not
mirroring) with functionality exposed via the new CefMediaRouter interface.
To test: Navigate to http://tests/media_router in cefclient and follow the
on-screen instructions.
