LibHTML: Implement the <br> element for line breaking

The <br> element will produce a special LayoutBreak node in the layout
tree, which forces a break in the line layout whenever encountered.

This patch also makes LayoutBlock use the current line-height as the
minimum effective height for each line box. This ensures that having
multiple <br> elements in a row doesn't create 0-height line boxes.

Libraries/LibHTML/DOM/HTMLBRElement.cpp

@@ -0,0 +1,16 @@
+#include <LibHTML/DOM/HTMLBRElement.h>
+#include <LibHTML/Layout/LayoutBreak.h>
+
+HTMLBRElement::HTMLBRElement(Document& document, const String& tag_name)
+    : HTMLElement(document, tag_name)
+{
+}
+
+HTMLBRElement::~HTMLBRElement()
+{
+}
+
+RefPtr<LayoutNode> HTMLBRElement::create_layout_node(const StyleResolver&, const StyleProperties*) const
+{
+    return adopt(*new LayoutBreak(*this));
+}