Rename RequestFinishedInfo.Metrics.getResponseEnd() to getRequestEnd()
The timestamp is available even when there is no response, so this name
makes more sense.
BUG=
629194,
653661
TBR=mef@chromium.org
NOPRESUBMIT=true
NOTRY=true
Review-Url:
https://codereview.chromium.org/2401933002
Cr-Commit-Position: refs/heads/master@{#423872}
(cherry picked from commit 1ad34d61a5dc3b4415c9448885a36e8f442b2c0b)