Re: [mediacapture-main] Prompt user to choose unless constraints reduce to 1. (#644)

To clarify this PR: I believe these changes to `getUserMedia` are the best path toward it becoming useful as an in-chrome picker.

It should be highly web compatible since most users won't see a change. https://github.com/w3c/mediacapture-main/pull/644#issuecomment-561253297

From [slide](https://docs.google.com/presentation/d/1QEdMf6Ixg1NNvHoZUvO29njrSKn9pdF1WqcrFWI4uX8/edit?ts=5dbaf0b6#slide=id.g6b7f95ec98_3_139):


User with previously persisted permission for camera: | Single camera | Front/back camera | Multiple cameras
-- | -- | -- | --
getUserMedia({video: true}) getUserMedia({video: {facingMode: "user"}}) getUserMedia({video: {deviceId: cameraId}}) | Granted! | _**Selector!**_ | _**Selector!**_
gUM({video: {facingMode: {exact: "user"}}}) | Granted! | Granted! | _**Selector!**_
gUM({video: {deviceId: {exact: cameraId}}}) | Granted! | Granted! | Granted!

**User without persisted permission:** replace “Granted!” with “Selector!” or “Prompt!”


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Received on Monday, 16 December 2019 21:20:01 UTC