[Mac] Passwords: Don't always prompt to access passwords saved by other browsers

Disable Keychain password access prompts when Chrome's "Ask to save passwords"
option is disabled. This gives users an escape hatch other than clicking
"Always allow" or "Deny" for each password prompt when they have passwords
saved from other browsers. This previously regressed with http://crrev.com/166878.

R=stuartmorgan@chromium.org
TBR=atwilson@chromium.org
BUG=178358
TEST=
1. In *Safari*, navigate to a login page for which you can save a password.
2. Log in, and instruct Safari to save the password.
3. In *Chrome*, under Advanced Settings, disable password management.
4. Navigate to the same login page.
At step (4), there should be no prompt for authorization to access the
password saved in Safari.

Review URL: https://codereview.chromium.org/106053008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241255 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241255