Summary of action item:
Move ownership of pending navigation item from NavigationManager to the NavigationContext. This will ensure that that the correct NavigationItem can always be found in any WKNavigationDelegate callback. The NavigationManager should store a pointer (non-owned) to the pending NavigationContext to look up details related to the pending navigation item.

Postmortem link: https://docs.google.com/document/d/1fPylc7WQEClKhI_1HkrKUwXShJvk0UwG5Gj0TxFk9oA/edit