LibWeb: Add a line box fragment for <br> nodes

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.

Userland/Libraries/LibWeb/Layout/BreakNode.cpp

@@ -42,7 +42,8 @@ BreakNode::~BreakNode()
 
 void BreakNode::split_into_lines(InlineFormattingContext& context, LayoutMode)
 {
-    context.containing_block().add_line_box();
+    auto& line_box = context.containing_block().add_line_box();
+    line_box.add_fragment(*this, 0, 0, 0, context.containing_block().line_height());
 }
 
 }