NT should handle multi-request fetches

[plehegar]

The intent behind w3c/resource-timing#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 w3c/resource-timing#21, there may be multiple PerformanceNavigationTiming records (e.g. one or more redirect requests plus final 200 OK req/resp).