As discussed during meeting I will split lock and login metrics for the shelf.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/513a946d686ed409a10a625d061c384b096fd555

commit 513a946d686ed409a10a625d061c384b096fd555
Author: Aga Wronska <agawronska@chromium.org>
Date: Wed Feb 07 00:59:12 2018

Add metrics for shelf button on Login screen.

Add new metrics for 'Browse as guest' and 'Add person' buttons,
that are only shown on login screen shelf.
Split existing click target metrics into separate histograms for
login and lock screen.
Use helper enumerations that are converted to histogram values
to keep metrics deduction code enclosed in metrics recorders.

Bug:  798848 
Change-Id: I9a6ddbccfcd301dec770362916ceaa7ca9fa721e
Reviewed-on: https://chromium-review.googlesource.com/894063
Commit-Queue: Aga Wronska <agawronska@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Xiaoyin Hu <xiaoyinh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534859}
[modify] https://crrev.com/513a946d686ed409a10a625d061c384b096fd555/ash/login/ui/note_action_launch_button.cc
[modify] https://crrev.com/513a946d686ed409a10a625d061c384b096fd555/ash/metrics/login_metrics_recorder.cc
[modify] https://crrev.com/513a946d686ed409a10a625d061c384b096fd555/ash/metrics/login_metrics_recorder.h
[modify] https://crrev.com/513a946d686ed409a10a625d061c384b096fd555/ash/metrics/login_metrics_recorder_unittest.cc
[modify] https://crrev.com/513a946d686ed409a10a625d061c384b096fd555/ash/metrics/user_metrics_recorder.cc
[modify] https://crrev.com/513a946d686ed409a10a625d061c384b096fd555/ash/metrics/user_metrics_recorder.h
[modify] https://crrev.com/513a946d686ed409a10a625d061c384b096fd555/ash/shelf/login_shelf_view.cc
[modify] https://crrev.com/513a946d686ed409a10a625d061c384b096fd555/ash/system/ime_menu/ime_menu_tray.cc
[modify] https://crrev.com/513a946d686ed409a10a625d061c384b096fd555/ash/system/tray/system_tray.cc
[modify] https://crrev.com/513a946d686ed409a10a625d061c384b096fd555/ash/system/virtual_keyboard/virtual_keyboard_tray.cc
[modify] https://crrev.com/513a946d686ed409a10a625d061c384b096fd555/ash/system/web_notification/web_notification_tray.cc
[modify] https://crrev.com/513a946d686ed409a10a625d061c384b096fd555/tools/metrics/histograms/enums.xml
[modify] https://crrev.com/513a946d686ed409a10a625d061c384b096fd555/tools/metrics/histograms/histograms.xml