WebIDL doesn't have a concept of "overloading", where a method is declared in an interface and has new steps in a subclass that inherits from it.
The proper way to do this in WebIDL is to redefine that method in the sub-interface, and define new steps for it.

This is true for PerformanceNavigationTiming that inherits from PerformanceResourceTiming, but also for all the other types of entries that inherit from PerformanceEntry.