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Here we fix few password save flaws:
1. Invalid password save offer when a password form was filled and then a non
password form was submitted.
2. A proper password form handling when a form is submitted into about:blank
iframe using "target" attribute. This is a valid flow which some websites use to send private information via iframe.

The iframe test is implemented exactly for the #2 flaw.

Bug: 854171
Change-Id: I39aec0d9b1f5fcbda551efb145f0c3be2456c0da
Reviewed-on: https://chromium-review.googlesource.com/1106137
Commit-Queue: Alexey Khoroshilov <sense@yandex-team.ru>
Reviewed-by: Vadym Doroshenko <dvadym@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575239}