layout: Store table parts in DOM layout data (#36447)

When laying out tables, store the boxes of non-anonymous table parts in
their respective DOM objects. This is going to be important for
incremental layout, but also for mapping from the DOM to the box tree
(and eventually the fragment tree).

For now, anonymous table parts are still lost to time and space, but
in a followup change we hope to store them somewhere.

Testing: This has no visible change to web rendering, so is covered by
existing
WPT.

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

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

components/layout_2020/flow/root.rs

@@ -212,6 +212,7 @@ impl BoxTree {
                         },
                         _ => return None,
                     },
+                    LayoutBox::TableLevelBox(..) => return None,
                     LayoutBox::TaffyItemBox(taffy_level_box) => match &taffy_level_box
                         .borrow()
                         .taffy_level_box