[webidl] Make the length attribute mandatory for interfaces that support indexed properties (#80)

Currently, interfaces that support indexed properties only make the length property mandatory if they are iterable. The length property is also mentioned as a sufficient condition for the interface to have an `@@iterator` symbol.

It should probably be made crystal-clear that interfaces that support indexed properties should always have a length attribute.

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Reply to this email directly or view it on GitHub:
https://github.com/heycam/webidl/issues/80