contentcollector: Avoid `for..in` iteration of object properties
contentcollector: Avoid `for..in` iteration of object properties

`for..in` iterates over inherited properties, which is almost never
desired. In most cases there aren't any inherited enumerable
properties so it's not that big of a deal, but in the case of
HTMLCollection it's very bad because it iterates over every entry
twice (once by numerical index and once by name) plus it includes the
`length` property in the iteration.

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