File per discussion with Emilio in https://phabricator.services.mozilla.com/D96128.

This part of the code computing the offset near the end of RecomputePosition() in RestyleManager.cpp should be converted to logical math.

Some hints:

1. We should consider using the containing block's writing mode to do the math per "the positioning phase" in the spec https://drafts.csswg.org/css-writing-modes-3/#orthogonal-flows
2. The existing test cases for this part of the code are in layout/reftests/position-dynamic-changes, but none of them have writing-mode.