layout: Collect both start and end baselines for fragments (#31230)

This change starts collecting the starting baseline set for fragments,
which is necessary for some layout modes (flex and tables, namely) as
well as being important for the implementation of `align-items`. In
addition, it converts baseline measurement to use `Au` everywhere.

Co-authored-by: Oriol Brufau <obrufau@igalia.com>

components/layout_2020/flow/float.rs

@@ -982,7 +982,6 @@ impl FloatBox {
                     // Clearance is handled internally by the float placement logic, so there's no need
                     // to store it explicitly in the fragment.
                     None, // clearance
-                    None, // last_inflow_baseline_offset
                     CollapsedBlockMargins::zero(),
                 )
             },