LibHTML: Add LayoutNode::document() for easy access

Every LayoutNode indirectly belongs to some Document. For anonymous
LayoutNodes, we simply traverse the parent chain until we find someone
with a Node from which we can get a Document&.

Libraries/LibHTML/Layout/LayoutNode.cpp

@@ -55,3 +55,10 @@ HitTestResult LayoutNode::hit_test(const Point& position) const
     });
     return result;
 }
+
+const Document& LayoutNode::document() const
+{
+    if (is_anonymous())
+        return parent()->document();
+    return node()->document();
+}