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/Layout/LayoutBlock.cpp

@@ -54,10 +54,12 @@ void LayoutBlock::layout_inline_children()
         child.split_into_lines(*this);
     });
 
+    int min_line_height = style().line_height();
+
     int content_height = 0;
 
     for (auto& line_box : m_line_boxes) {
-        int max_height = 0;
+        int max_height = min_line_height;
         for (auto& fragment : line_box.fragments()) {
             max_height = max(max_height, fragment.rect().height());
         }

Libraries/LibHTML/Layout/LayoutBreak.cpp

@@ -0,0 +1,17 @@
+#include <LibHTML/Layout/LayoutBlock.h>
+#include <LibHTML/Layout/LayoutBreak.h>
+
+LayoutBreak::LayoutBreak(const HTMLBRElement& element)
+    : LayoutNode(&element)
+{
+    set_inline(true);
+}
+
+LayoutBreak::~LayoutBreak()
+{
+}
+
+void LayoutBreak::split_into_lines(LayoutBlock& block)
+{
+    block.line_boxes().append(LineBox());
+}

Libraries/LibHTML/Layout/LayoutBreak.h

@@ -0,0 +1,16 @@
+#pragma once
+
+#include <LibHTML/DOM/HTMLBRElement.h>
+#include <LibHTML/Layout/LayoutNode.h>
+
+class LayoutBreak final : public LayoutNode {
+public:
+    explicit LayoutBreak(const HTMLBRElement&);
+    virtual ~LayoutBreak() override;
+
+    const HTMLBRElement& node() const { return to<HTMLBRElement>(*LayoutNode::node()); }
+
+private:
+    virtual const char* class_name() const override { return "LayoutBreak"; }
+    virtual void split_into_lines(LayoutBlock&) override;
+};