Add unit test for password filling logic

Bug: 865114
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I5617f95e83c661d046e1a9d148362c2b436d2240
Reviewed-on: https://chromium-review.googlesource.com/1195292
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Commit-Queue: Scott Wu <scottwu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588528}
7 files changed