The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/69375efc1e017aacc17871db4c17a73948208b69

commit 69375efc1e017aacc17871db4c17a73948208b69
Author: Alex Newcomer <newcomer@chromium.org>
Date: Tue Nov 13 00:35:06 2018

cros: Add kTabletMode as an enum for the histogram

When the tablet mode transition was added to Apps.AppListShowSource,
the enum was not updated. Just add a third enum entry.

Bug:  904131 
Change-Id: Icaee0f359b42621381c19f8d5cfe9a62ef2cd51d
Reviewed-on: https://chromium-review.googlesource.com/c/1331034
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Reviewed-by: Weidong Guo <weidongg@chromium.org>
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607391}
[modify] https://crrev.com/69375efc1e017aacc17871db4c17a73948208b69/tools/metrics/histograms/enums.xml