Implement .iter to DOMRectList

[jaysonsantos]

This will be used in #10828 but I made thins in a splited PR because the other one is pretty hard to get working and this iterator still would be useful.

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This change is

[KiChjang]

Why don't you implement Iterator directly for DOMRectList instead?

[cbrewster]

Iter stores the current position, but perhaps it would be better to rename Iter to IterableDOMRectList or DOMRectListIterator

[jaysonsantos]

@KiChjang I though that but it would mutate the DOMRectList and it would only be iterated once because of position. Does it make sense?
@ConnorGBrewster I used the same naming as in the std, is better to change it?

[KiChjang]

Actually, the better question here is why not simply return self.rects.iter() in fn iter.

[jaysonsantos]

I am maitaining Item signature which is Root<DOMRect> and self.rects is a vector of JS<DOMRect>

[KiChjang]

self.rects.iter().map(|rect| Root::from_ref(&*rect))?

[Ms2ger]

You can't return that from a function. However, it would probably be better to store self.rects.iter() in a field of Iter and then return self.inner.next().map(|rect| Root::from_ref(&*rect)) from next().

[nox]

I don't understand, why do we need a specific iterator instead of just returning a &[&DOMRect]?

[jaysonsantos]

@nox I don't know either :) I am just starting at this business. But to return it I would have to iterate over all rects to make it, no? And it wouldn't be lazy.

[KiChjang]

Superseded by #11171.