CL https://codereview.chromium.org/2902093003/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/eda5652c60377024288d24f9ebc02b42619ebe9d

commit eda5652c60377024288d24f9ebc02b42619ebe9d
Author: kinaba <kinaba@chromium.org>
Date: Thu May 25 00:02:12 2017

arc: Message for a low-battery case of migration notification.

BUG= 710289 
TEST=Manually run on 29% and 31% battery devices.

Review-Url: https://codereview.chromium.org/2902093003
Cr-Commit-Position: refs/heads/master@{#474483}

[modify] https://crrev.com/eda5652c60377024288d24f9ebc02b42619ebe9d/chrome/browser/chromeos/BUILD.gn
[add] https://crrev.com/eda5652c60377024288d24f9ebc02b42619ebe9d/chrome/browser/chromeos/arc/arc_migration_constants.h
[modify] https://crrev.com/eda5652c60377024288d24f9ebc02b42619ebe9d/chrome/browser/chromeos/arc/arc_migration_guide_notification.cc
[modify] https://crrev.com/eda5652c60377024288d24f9ebc02b42619ebe9d/chrome/browser/ui/webui/chromeos/login/encryption_migration_screen_handler.cc

Verification steps:
0. Keep the battery percentage around 20%.
1. Flash Kevin R59. Log in with an account.
2. AU to M60. Re-login to the account.
3. Press "skip" in the "critical update" migration screen.
4. Press Play Store icon in the launcher.

Expected: Notification message mentions that the user should *charge* and then update.