The presentation request should be the default cast mode, correct?  Sometimes the user wants to cast but knows that the site provides a poor experience via the Presentation API.  In this case, we want to make the mirroring modes easily discoverable.

I was investigating the state of our web platform tests and ran into a surprise.  Right now the Presentation API behavior is not ideal (and not to spec).

If you can new PresentationRequest("http://yahoo.com").start(), the dialog will appear with mirroring devices.   If the user selects one, MR will mirror the *current tab*, not yahoo.com, as we don't have 1-UA mode.  That's not what the user intended.

The spec says that we should reject if there are no presentation displays capable of displaying the given URL.

Here is what I think should happen:
- If availability = false for the PresentationRequest, then reject
- If availability = true, then open the dialog with default cast mode from the request and only sinks listed that can present the URL
- If availability is unresolved, open the dialog with an empty sink list.

To handle the case where we haven't gotten the initial sink list for the request, we should make sure the MR is not sending an empty sink list (and setting availability to false) before getting a query result from the MRPM.

In the case in comment #2, the URL is presentable (via Cast), but the user may prefer to mirror even when initiating casting from the page.  I think that will be covered by sticky override behavior.

Implementation notes:

- Ideally the rejection behavior can be implemented in Blink and doesn't require lots of plumbing; however, it looks like there is no direct reference from a PresentationRequest to PresentationAvailability, so hmmm.

- The logic in MediaRouterDialogController will need some tweaks it looks like to configure the dialog specifically for PresentationRequests.

With the imminent launch of Presentation API 1-UA mode, it will be possible to present URLs to mirroring targets.