[Resource Timing] Browser discrepancies with requests that don't return a response

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.

I think the browser should include these entries and that the spec needs
updating to make the expected behaviour explicit.

The behaviour can be replicated using
http://www.co-operativeinsurance.co.uk/petinsurance/cat-insurance as it
includes an resource from 10.64.30.186

I checked it in the following browser versions:

FF Nightly 38.0a1 (2015-02-04)
IE 11.0.9600.17501
Chrome 40.0.2214.111
Canary 42.0.2308.0

I'll raise a crbug too.

Cheers

Andy