LibWeb: Whine in debug log instead of asserting on partial layout FIXME

We don't support incremental relayout of subtrees (only single nodes)
but let's not crash the browser just because this happens. We can keep
the browser up and just complain in the debug log instead.

Libraries/LibWeb/Layout/LayoutTreeBuilder.cpp

@@ -79,8 +79,10 @@ static RefPtr<LayoutNode> create_layout_tree(Node& node, const StyleProperties*
 
 RefPtr<LayoutNode> LayoutTreeBuilder::build(Node& node)
 {
-    // FIXME: Support building partial trees.
-    ASSERT(is<Document>(node) || !node.has_children());
+    if (!is<Document>(node) && node.has_children()) {
+        dbg() << "FIXME: Support building partial layout trees.";
+        return nullptr;
+    }
     return create_layout_tree(node, nullptr);
 }