Re: [mediacapture-main] Add semantics choice to getUserMedia() for in-browser picker. (#667)

**Summary of PR:** Based on consensus in February call (Option A/B in [slides](https://docs.google.com/presentation/d/1NIHzumglY9cYa4b7rcEbHGVsMam5BiY80VfFDB6cDjQ/edit#slide=id.g7ead6f3266_5_70)). New semantics:
```js
navigator.mediaDevices.getUserMedia({video: constraints, semantics: "user-chooses"});
```
...to work as follows:
 * Users with 1 device satisfying the constraints: permission prompt only if needed (like today). 
 * Users with >1 device satisfying the constraints: **_browser_** opens device picker always.

Sites avoid picker on revisits by remembering w/`deviceId` constraint to narrow constraints to 1.

Use this to replace today's in-content device pickers. May also be used on initial call.

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Received on Monday, 11 May 2020 12:56:44 UTC