Chrome OS sign-in: Remove WebUI login header bar.

Since header bar is now Views-only, this Cl removes its old WebUI version.
As a consequence it also removes all references to it and its contents.
Kiosk tests are migrated to Views shelf, new test APIs is added to support
this.

Login screen mojo API SetKioskApps is changed to return a dummy value to
be able to wait until it finishes.

TBR=ochang@chromium.org

Bug: 926638
Change-Id: Ib94eafff734960d6b0413c9b1bd7d7416427e7a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1469644
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Reviewed-by: Oliver Chang <ochang@chromium.org>
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: Wenzhao (Colin) Zang <wzang@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638928}
88 files changed