bugfix: Improve consistency of PerformanceEntries
    
    Without this bugfix, the repeated calls of the wrapped functions return
    different results. This is (a) fingerprintable, (b) provides confusing
    results.
    
    E.g. the startTime on https://www.fit.vutbr.cz/~ipolcak/firefox_csp_bug/nocsp.php
    increases even though it does not change.
    
    The wrapping now remembers previously returned modified values and
    reuses them for calls with the same parameters.