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This CL adds a button to the login-dev-overlay to pop up a fake kiosk
error message.

We also update the ToastManager to allow showing toasts on the Login
screen (which also uses LockSystemModalContainer). This involves
consolidating the toast requeueing logic into OnSessionStateChanged.

Bug: 879117
Change-Id: I557f0444a2368d1bf460d5db0a41f893ce0e69ea
Reviewed-on: https://chromium-review.googlesource.com/1211969Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Commit-Queue: Quan Nguyen <qnnguyen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589968}