The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5f7fd11473c746726e2e1917680a34acde058f19

commit 5f7fd11473c746726e2e1917680a34acde058f19
Author: yusukes <yusukes@chromium.org>
Date: Fri May 26 02:42:03 2017

Add Login.PromptToLoginTime UMA metric

This is similar to Login.PromptToCompleteLoginTime, but for existing
users.

BUG= 726345 
TEST=manually confirmed that the path is taken when an existing
 user signs into Chrome OS

Review-Url: https://codereview.chromium.org/2902373002
Cr-Commit-Position: refs/heads/master@{#474885}

[modify] https://crrev.com/5f7fd11473c746726e2e1917680a34acde058f19/chrome/browser/chromeos/login/existing_user_controller.cc
[modify] https://crrev.com/5f7fd11473c746726e2e1917680a34acde058f19/tools/metrics/histograms/histograms.xml