Verify TTFB in resource-timing.html WPT by measuring that the delay occurs between responseStart and responseEnd

When simulating a delay after a statusline, measure that the delay is
not included in the TTFB by measuring that it occurred between
responseStart and responseEnd rather than requestStart and
responseStart. This makes the tests less flaky when subject to shaky
network and thread scheduling conditions, as might be present using
Firefox's chaos mode.

Bug: 568024
Change-Id: I0356f6256af6bc87fbb14860ed05472526c03e5d
Reviewed-on: https://chromium-review.googlesource.com/c/1377725
Reviewed-by: Yoav Weiss <yoav@yoav.ws>
Commit-Queue: Andrew Comminos <acomminos@fb.com>
Cr-Commit-Position: refs/heads/master@{#617021}
1 file changed