layout: Cache SameFormattingContextBlock layout results (#42904)

Add a cache for `SameFormattingContextBlock`s: A new kind of cache
is added which allows reusing fragments for
`SameFormattingContextBlock` (the majority of block level elements).
This can reduce the amount of fragment tree construction during
incremental layout (although with a 40 byte overhead per
`SameFormattingContextBlock`).

Testing: This change adds a new Servo-specific WPT test that verifies
that
the minimal number of `Fragment`s are rebuilt after an incremental
layout.
Previously, when running `flexbox-deeply-nested-column-flow.html` from
`tests/blink_perf_tests/perf_tests/layout/`, performance would degrade
with successive test runs. This is due to how slow inline layout is.
This change
removes that progressive degradation, though we may be able to remove
this cache with improvements to inline layout.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Luke Warlow <lwarlow@igalia.com>
Co-authored-by: Oriol Brufau <obrufau@igalia.com>

tests/wpt/mozilla/meta/MANIFEST.json

@@ -13159,6 +13159,13 @@
       null,
       {}
      ]
+    ],
+    "preserved-same-formatting-context-block-layout.html": [
+     "068936574cedc2ad55a98f8a24ea419fab3535cf",
+     [
+      null,
+      {}
+     ]
     ]
    },
    "input-events": {