mac: Don't show keychain prompt for password Autofill.

mac: Don't show keychain prompt for password Autofill.

Password Autofill is supposed to be a convenience. If it creates a
blocking dialog, it is no longer convenient. The original intention of
password Autofill was that it would only prompt the user after a full
username was typed in. Until that behavior is implemented, never prompt the
user for keychain access.

Effectively, this means that passwords stored by Chrome still work, since
Chrome can access those without a prompt, but passwords stored by Safari,
Firefox, or Chrome Canary will not work. Note that the latest build of Safari
and Firefox don't create keychain items with the relevant tags anyways.

BUG=178358

Review URL: https://codereview.chromium.org/386043005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@283805 0039d316-1c4b-4281-b951-d872f2087c98