The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4dbf36b9d88184677aa4aac9feb2f71686af7e1c

commit 4dbf36b9d88184677aa4aac9feb2f71686af7e1c
Author: Weidong Guo <weidongg@chromium.org>
Date: Thu Jan 18 23:38:35 2018

Add UMA to display mirror mode

Changes:
Add metric to record the time used to process SetMirrorMode API, types
of display mirror mode and ranges of the number of displays in mirror
mode.

Bug:  802311 
Change-Id: Icad93a833ec23a4aab2ffbfeda2e20d632c05b5e
Reviewed-on: https://chromium-review.googlesource.com/868491
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Weidong Guo <weidongg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#530340}
[modify] https://crrev.com/4dbf36b9d88184677aa4aac9feb2f71686af7e1c/tools/metrics/histograms/enums.xml
[modify] https://crrev.com/4dbf36b9d88184677aa4aac9feb2f71686af7e1c/tools/metrics/histograms/histograms.xml
[modify] https://crrev.com/4dbf36b9d88184677aa4aac9feb2f71686af7e1c/ui/display/manager/display_manager.cc