cros: Do not store unowned ExistingUserController instances.

LoginDisplayHost owns ExistingUserController, this makes it more obvious and
also simplifies instance creation.

Bug: 851012
Change-Id: I8dc24c743aeb71070801009b2b4ec6472ee16409
Reviewed-on: https://chromium-review.googlesource.com/1125029
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Jacob Dufault <jdufault@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575655}
13 files changed