(Set)IsOverridingUserAgent should be called on VisibleItem

ios/web/navigation/navigation_manager_impl.mm

Index: ios/web/navigation/navigation_manager_impl.mm
diff --git a/ios/web/navigation/navigation_manager_impl.mm b/ios/web/navigation/navigation_manager_impl.mm
index 140c760468abf0f45fe5fdaa6d512dbb8b08ed32..e1161720915f53bf11a578df9041e4051f72b097 100644
--- a/ios/web/navigation/navigation_manager_impl.mm
+++ b/ios/web/navigation/navigation_manager_impl.mm
@@ -420,6 +420,10 @@ bool AreURLsInPageNavigation(const GURL& existing_url, const GURL& new_url) {
   return result;
 }
 
+bool NavigationManagerImpl::GetUseDesktopUserAgent() const {
+  return GetVisibleItem() && GetVisibleItem()->IsOverridingUserAgent();
+}
+
 void NavigationManagerImpl::OverrideDesktopUserAgentForNextPendingItem() {
   if (GetPendingItem())
     GetPendingItem()->SetIsOverridingUserAgent(true);