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.cpp

@@ -0,0 +1,31 @@
+#include <LibHTML/DOM/Document.h>
+#include <LibHTML/Frame.h>
+
+Frame::Frame()
+{
+}
+
+Frame::~Frame()
+{
+}
+
+void Frame::set_document(Document* document)
+{
+    if (m_document == document)
+        return;
+
+    if (m_document)
+        m_document->detach_from_frame({}, *this);
+
+    m_document = document;
+
+    if (m_document)
+        m_document->attach_to_frame({}, *this);
+}
+
+void Frame::set_size(const Size& size)
+{
+    if (m_size == size)
+        return;
+    m_size = size;
+}