[ios] Remove use of rootViewController from autofill

[UIApplication sharedApplication].keyWindow.rootViewController was
used to present UI. Now, the appropriate view controller is
plumbed into autofill to present UI.

Bug: 791793, 692525
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: If82b5a5ee37d2ea9778d0332dd6686949d6bbfab
Reviewed-on: https://chromium-review.googlesource.com/882400
Reviewed-by: Mark Cogan <[email protected]>
Reviewed-by: Moe Ahmadi <[email protected]>
Reviewed-by: edchin <[email protected]>
Cr-Commit-Position: refs/heads/master@{#531991}
11 files changed