Auto merge of #29946 - Loirooriol:clearance-option, r=mrobinson

Layout 2020: implement clearance as Option<Length>

Clearance was implemented as a Length, where zero meant no clearance. However, having a clearance of 0px should be different than having no clearance, since the former can still prevent margin collapse.

This patch keeps the existing behavior, so it won't be possible to get a clearance of Some(Length::zero()), but it prepares the terrain for a follow-up to fix calculate_clearance to return the proper thing.

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