Signin: Rename Chrome OS-only user selection screen to chromeos_ prefix.

md_* files in WebUI sigin are only used on Chrome OS.
This Cl changes the prefix to chromeos_ to avoid confusion.

Bug: 926638

Change-Id: I17e006e401d48d36466e8582e6bbe3efc40f0566
Commit-Queue: Alexander Alekseev <>
Reviewed-by: Alexander Alekseev <>
Reviewed-by: Jacob Dufault <>
Reviewed-by: Wenzhao (Colin) Zang <>
Cr-Commit-Position: refs/heads/master@{#632578}
15 files changed