https://lists.w3.org/Archives/Public/public-web-perf/2015Feb/0065.html

There's some differences in the way browsers treat requests that don't
return a response. FF Nightly and IE 11 both create PerformanceResourceTiming entries for
these requests, whereas Chrome & Canary don't. The RT spec isn't explicit as to whether requests that miss a response should be included. -- @andydavies