To support some new UMA histograms (bug 877972) and some new PerformanceResourceTiming attributes ( bug 875013 ) we need some more data about how a Response was produced.  Currently FetchAPIResponse has:

 * cache_storage_name
 * is_in_cache_storage
 * response_time which tracks original fetch time

It would be nice to have information showing:

 * produced from fetch() via network
 * produced from fetch() via http cache
 * produced from new Response()
 * creation start time corresponding to start of fetch(), cache.match(), etc
 * creation end time corresponding to fetch(), cache.match(), etc promise settling