LibWeb: Resolve two document-tree child browsing context count FIXMEs

This patch makes use of helpers implemented for window.length to resolve
two FIXMEs in WindowProxy previously simply assuming no child browsing
contexts :^)

Author: networkException

Userland/Libraries/LibWeb/Bindings/WindowProxy.cpp

@@ -72,8 +72,8 @@ JS::ThrowCompletionOr<Optional<JS::PropertyDescriptor>> WindowProxy::internal_ge
         // 1. Let index be ! ToUint32(P).
         auto index = property_key.as_number();
 
-        // FIXME: 2. Let maxProperties be the number of document-tree child browsing contexts of W.
-        size_t max_properties = 0;
+        // 2. Let maxProperties be the number of document-tree child browsing contexts of W.
+        auto max_properties = TRY(m_window->document_tree_child_browsing_context_count());
 
         // 3. Let value be undefined.
         Optional<JS::Value> value;
@@ -227,8 +227,8 @@ JS::ThrowCompletionOr<JS::MarkedVector<JS::Value>> WindowProxy::internal_own_pro
     // 2. Let keys be a new empty List.
     auto keys = JS::MarkedVector<JS::Value> { vm.heap() };
 
-    // FIXME: 3. Let maxProperties be the number of document-tree child browsing contexts of W.
-    size_t max_properties = 0;
+    // 3. Let maxProperties be the number of document-tree child browsing contexts of W.
+    auto max_properties = TRY(m_window->document_tree_child_browsing_context_count());
 
     // 4. Let index be 0.
     // 5. Repeat while index < maxProperties,