[Password Generation] Use signatures for form matching

At this time, the form matching is based on form name and action. Action should have canonical form and be non-empty. We missed to prepare actions in some cases. So, even if we classified a field as suitable for password generation, we didn't show generation popup, because form matching failed. It should be fixed.

Since the server sends signatures of form and fields, it is more error-prone to match forms based on what server works with.


Review-Url: https://codereview.chromium.org/2318533002
Cr-Commit-Position: refs/heads/master@{#420043}
33 files changed