layout: Store Fragment results in LayoutBoxBase and start using them for queries (#36583)

Start storing a link to laid-out `Fragment`s in `LayoutBoxBase`, so that
these are accessible for queries and eventually for incremental layout.
Some box tree data structures lacked a `LayoutBoxBase`, such as table
tracks and table track groups[^1].

In addition, start using these `Fragment`s for queries instead of
walking the entire `Fragment` tree. Currently, this isn't possible for
most queries as `Fragment`s do not cache their absolute offsets (which
are often necessary). This change uses the new box tree `Fragment`s for
most resolved style queries.

[^1]: Note that only rows and row groups store `Fragment`s as columsn
and
   colgroups do not produce any.

Testing: This is covered by existing tests.
Fixes: This is part of #36525.

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

components/layout_2020/taffy/mod.rs

@@ -131,6 +131,17 @@ impl TaffyItemBox {
             },
         }
     }
+
+    pub(crate) fn fragments(&self) -> Vec<Fragment> {
+        match self.taffy_level_box {
+            TaffyItemBoxInner::InFlowBox(ref independent_formatting_context) => {
+                independent_formatting_context.base.fragments()
+            },
+            TaffyItemBoxInner::OutOfFlowAbsolutelyPositionedBox(ref positioned_box) => {
+                positioned_box.borrow().context.base.fragments()
+            },
+        }
+    }
 }
 
 /// Details from Taffy grid layout that will be stored