Re: [mediacapture-region] Refer to getBoundingClientRect as a defintion of the actual crop area (#32)

I think the following is simplest, as (1) it does not require reading multiple sections and trying to coordinate them, and (2) it does not mention "weak references", which could be misunderstood.

```
Let <var>element</var> be the Element from which CropTarget was constructed.
Calls to this method instruct the user agent to start/stop cropping a [=self-capture video track=]
to the <a href="https://drafts.csswg.org/cssom-view/#dom-element-getboundingclientrect">
bounding client rectangle</a> of <var>element</var>.
Since the track is restricted to the visible viewport of the [=display-surface=],
the captured area will be the intersection of the visible viewport and the element
bounding client rectangle.
```

For serialization, I think we could employ a similar approach. Possibly a slot would be useful there, possibly not; but here, I think this is simplest. Wdyt?

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