LibHTML: Add a Frame class and use it for document layout width

Each HtmlView now has a main_frame(), which represents the main frame
of the web page. Frame inherits from TreeNode<Frame>, which will allow
us to someday implement a Frame tree (to support the <frame> element.)

Libraries/LibHTML/Frame.h

@@ -0,0 +1,31 @@
+#pragma once
+
+#include <AK/Noncopyable.h>
+#include <AK/RefPtr.h>
+#include <AK/Weakable.h>
+#include <LibDraw/Size.h>
+#include <LibHTML/TreeNode.h>
+
+class Document;
+
+class Frame
+    : public TreeNode<Frame>
+    , public Weakable<Frame> {
+public:
+    static NonnullRefPtr<Frame> create() { return adopt(*new Frame); }
+    ~Frame();
+
+    const Document* document() const { return m_document; }
+    Document* document() { return m_document; }
+
+    void set_document(Document*);
+
+    const Size& size() const { return m_size; }
+    void set_size(const Size&);
+
+private:
+    Frame();
+
+    RefPtr<Document> m_document;
+    Size m_size;
+};