This is required for the block formatting context to know the height of
the <br> element while computing the height of its parent. Specifically,
this comes into play when the <br> element is the first or last child of
its parent. In that case, it previously would not have any fragments, so
BlockFormattingContext::compute_auto_height_for_block_level_element
would infer its top and bottom positions to be 0.
