LibWeb: Make factory method of DOM::TreeWalker fallible

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -1953,7 +1953,7 @@ JS::NonnullGCPtr<NodeIterator> Document::create_node_iterator(Node& root, unsign
 // https://dom.spec.whatwg.org/#dom-document-createtreewalker
 JS::NonnullGCPtr<TreeWalker> Document::create_tree_walker(Node& root, unsigned what_to_show, JS::GCPtr<NodeFilter> filter)
 {
-    return TreeWalker::create(root, what_to_show, filter);
+    return TreeWalker::create(root, what_to_show, filter).release_value_but_fixme_should_propagate_errors();
 }
 
 void Document::register_node_iterator(Badge<NodeIterator>, NodeIterator& node_iterator)

Userland/Libraries/LibWeb/DOM/TreeWalker.cpp

@@ -39,12 +39,12 @@ void TreeWalker::visit_edges(Cell::Visitor& visitor)
 }
 
 // https://dom.spec.whatwg.org/#dom-document-createtreewalker
-JS::NonnullGCPtr<TreeWalker> TreeWalker::create(Node& root, unsigned what_to_show, JS::GCPtr<NodeFilter> filter)
+WebIDL::ExceptionOr<JS::NonnullGCPtr<TreeWalker>> TreeWalker::create(Node& root, unsigned what_to_show, JS::GCPtr<NodeFilter> filter)
 {
     // 1. Let walker be a new TreeWalker object.
     // 2. Set walker’s root and walker’s current to root.
     auto& realm = root.realm();
-    auto walker = realm.heap().allocate<TreeWalker>(realm, root).release_allocated_value_but_fixme_should_propagate_errors();
+    auto walker = MUST_OR_THROW_OOM(realm.heap().allocate<TreeWalker>(realm, root));
 
     // 3. Set walker’s whatToShow to whatToShow.
     walker->m_what_to_show = what_to_show;

Userland/Libraries/LibWeb/DOM/TreeWalker.h

@@ -15,7 +15,7 @@ class TreeWalker final : public Bindings::PlatformObject {
     WEB_PLATFORM_OBJECT(TreeWalker, Bindings::PlatformObject);
 
 public:
-    static JS::NonnullGCPtr<TreeWalker> create(Node& root, unsigned what_to_show, JS::GCPtr<NodeFilter>);
+    static WebIDL::ExceptionOr<JS::NonnullGCPtr<TreeWalker>> create(Node& root, unsigned what_to_show, JS::GCPtr<NodeFilter>);
 
     virtual ~TreeWalker() override;