Converting to CSSPixels caused us to lose precision and the sign of
signed zeroes.
The values we resolve against in Length::ResolutionContext are still
themselves rounded too early but this is in the right direction.
When position changes, we may need to make larger structural updates
to the layout tree. A simple relayout is not sufficient.
This was a source of flakiness in the engine, and gives us at least
+28 new WPT subtest passes.
Math functions like abs(), clamp(), round(), etc, can be used by
themselves in property values, without wrapping them in calc().
Before this change, we were neglecting to run calc simplification on the
generated calculation node trees. By doing that manually after parsing a
standalone math function, we score at least a couple hundred WPT points.
