Remove ash dependency on WebUILoginView::MoveFocusToSystemTray

LoginScreen::FocusLoginShelf() mojo API can be reused.

BUG=854346
TEST=trybot

Change-Id: Ide7dad98e1da57826d37d5dff4c3e269aee07c14
Reviewed-on: https://chromium-review.googlesource.com/c/1325689
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Commit-Queue: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606662}
4 files changed