Auto merge of #29720 - Loirooriol:backport-D143942, r=mrobinson

Don't go through app-units-ratio to convert absolute lengths

This is a backport of https://phabricator.services.mozilla.com/D143942, by Emilio Cobos Álvarez.

This doesn't change behavior on its own, but the current code introduces some minor floating point error which we can avoid, and which would cause failures with the patch for #29696.

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