NT should handle multi-request fetches

[plehegar]

The intent behind https://github.com/w3c/resource-timing/issues/21 will break: [[ Only the current document resource gets included as the only PerformanceNavigationTiming object in the Performance Timeline of the relevant context. ]]

Assuming we land https://github.com/w3c/resource-timing/issues/21, there may be multiple PerformanceNavigationTiming records (e.g. one or more redirect requests plus final 200 OK req/resp).