[IOS Password Manager] Async suggestions retrieval.

When a password form is added dynamically (i.e. with JavaScript after finishing of a page load)
IOS Password Manager may learn about existing of this form only after user clicking on
some field in this form. On CL https://chromium-review.googlesource.com/c/chromium/src/+/601969
it was implemented retrieving credentials from the Password Store as soon as the user clicked
on dynamically added form and then credentials are autofilled. But fill on account select is not
available until the user makes another interaction with the form (for example start typing of
username). That's suboptimal user experience, it would be good to propose fill on
account select immediately as soon as the user clicks the first time in the form.

This CL implements async suggestion response to Autofill framework in the same way as it's
already implemented in Autofill:
1.When checkIfSuggestionsAvailableForForm (password_controller.mm) is called, and it's
found that credentials haven't been retrieved yet for this page, the callback is saved
untile response from the Password Store is received.
2.It's implemented that Password Store response is propagated to PasswordController even if case
when there are no credentials saved for this page in order to inform Autofill framework that
no suggestions are available.

Bug: 708597, 709011, 708634
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Idecc3e621100f3e1b0a3803ce944c4cc116fc3a4
Reviewed-on: https://chromium-review.googlesource.com/921961
Commit-Queue: Vadym Doroshenko <[email protected]>
Reviewed-by: Vasilii Sukhanov <[email protected]>
Cr-Commit-Position: refs/heads/master@{#540200}
7 files changed