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

TBR=dcheng@chromium.org

Change-Id: I17e006e401d48d36466e8582e6bbe3efc40f0566
Reviewed-on: https://chromium-review.googlesource.com/c/1468783
Commit-Queue: Alexander Alekseev <alemate@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Reviewed-by: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632578}
15 files changed