Improve password form matching.

Current code is written in a way that a form without action or name doesn't match itself. It's a performance problem because PasswordManager::CreatePendingLoginManagers continuously creates a PasswordFormManager for the same form leading to poor performance.
The code was firstly introduced in https://chromium-review.googlesource.com/c/chromium/src/+/620768. However, motivation for it is unclear after discussing together.

Bug: 801702,597309
Change-Id: Ie2458d8d5340852530675b9f5d7f627132e1f3c6
Reviewed-on: https://chromium-review.googlesource.com/946490
Commit-Queue: Vasilii Sukhanov <vasilii@chromium.org>
Reviewed-by: Vadym Doroshenko <dvadym@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541145}
7 files changed