Auto merge of #16239 - servo:cascade-collect, r=emilio
Auto merge of #16239 - servo:cascade-collect, r=emilio

Remove a memory allocation (`iter.collect::<Vec<_>>()`) in `cascade()`

This vector was there to pre-acquire locks and give all declarations the same lifetime (which is necessary for custom properties cascading).

https://github.com/servo/servo/pull/16014 introduce a guard to a shared pre-acquired lock, making this vector unnecessary.

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