[iOS][MF] Hide and show password icon

Hide password icon when no passwords are available and show it again
if passwords become available.

Updates SavePasswordsConsumer to post empty results and adds an early
return where this was expected to keep the same behavior.

Bug: 878388
Change-Id: Ie4948d3b68e1ce00fa34f37f9207d64fca174ceb
Reviewed-on: https://chromium-review.googlesource.com/c/1333427
Commit-Queue: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608094}