When a table row (or its group) is set to collapse, the row takes up no
vertical space in the layout.
We have to account for this in multiple places, so I've cached whether a
row is collapsed in the TableGrid::Row.
Instead, use the generic create_independent_formatting_context_if_needed
so that unusual situations like image-as-table-caption don't crash.
This logic clearly needs more work, but let's at least do better than
crashing. This gives us 26 new subtest passes on WPT.