ResourceTiming: Add finalResponseHeadersStart

As per WebPerfWG resolution:
- finalResponseHeadersStart is the timing of the 2xx/4xx/5xx response.
- responseStart is the timing of a 1xx response, if such exists, or else the timing of the final 2xx/4xx/5xx response.

Behind a flag: ResourceTimingFinalResponseHeadersStart
See chromestatus: https://chromestatus.com/feature/5158830722514944

Bug: 40251053
Change-Id: Ic48bef692502ab28dc0ddf2e7420b3a5763e2a6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5987770
Reviewed-by: Stefan Zager <szager@chromium.org>
Reviewed-by: Ian Clelland <iclelland@chromium.org>
Commit-Queue: Noam Rosenthal <nrosenthal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1377776}