This FR tracks the work for a new API to support desktop capture for Media Router (Cast).

The API is private as it will support a subset of features of the existing desktopCapture API and be whitelisted to the Media Router.

Proposal:

chrome.cast.desktopCapturePrivate.chooseScreenMedia(callback)
chrome.cast.desktopCapturePrivate.cancelChooseScreenMedia(requestId)

Takes: |callback| Invoked with an opaque stream id for the screen to be captured, or null if the request was cancelled or had an error.

Returns: An integer requestId used to cancel a pending request.

Behavior:
- If the user has a single screen media source, invokes |callback| immediately with the stream id.
- If the user has multiple sources, shows the same picker as chooseDesktopMedia with screen sources and allows the user to choose one.
- Audio capture is always requested and the audio checkbox is NOT shown in the picker.
- The title and messaging in the picker may be further tweaked for Cast, however, we are not going to go crazy here and reuse the existing picker code as much as possible.