[mediacapture-region] Pull Request: Update demo

beaufortfrancois has just submitted a new pull request for https://github.com/w3c/mediacapture-region:

== Update demo ==
This PR cleans up the demo used in this repository with latest spec changes:
- Use `cropTarget` instead of `cropID` everywhere
- Removal of Chrome origin trial tokens and the error text associated with it.
- Use `null` instead of the empty string `""` when uncropping

Nits:
- Remove superfluous `JSON.stringify()` when `postMessage()`
- Moves right.html `<video>` element at the top to avoid scrolling every time user wants to change sources.

@eladalon1983 Please have a look

See https://github.com/w3c/mediacapture-region/pull/66


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