LibHTML: Add a Frame class, start fleshing out recursive layout.

Layout is initiated from Frame::layout(). It makes the document's layout
node as wide as the frame, and then we'll take it from there.

LibHTML/Frame.h

@@ -0,0 +1,21 @@
+#pragma once
+
+#include <LibHTML/DOM/Document.h>
+#include <SharedGraphics/Size.h>
+
+class Frame {
+public:
+    Frame();
+    ~Frame();
+
+    Document* document() { return m_document.ptr(); }
+    const Document* document() const { return m_document.ptr(); }
+
+    void set_document(Document*);
+
+    void layout();
+
+private:
+    RetainPtr<Document> m_document;
+    Size m_size;
+};