[iOS][MF] Adds password fetcher for manual fill

This object will be in charge to fetch the passwords for Manual Fill.
A Passwords Mediator will be the delegate and owner of it.

Bug: 845472
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I7e15656c4d9979aff5c86d79bab7a42e5161408b
Reviewed-on: https://chromium-review.googlesource.com/1148813
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578337}
7 files changed