[ios clean] Move alert display to coordinator

The UI-spawning code should be in the coordinator, not in the mediator.

Bug: 753391
Change-Id: Ia85d0b00471598dd42e9b75c25cdb2b89968de03
Reviewed-on: https://chromium-review.googlesource.com/610055
Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#493706}
3 files changed