[Mac] Add option to reauthenticate the OS user before revealing passwords.

When the flag is enabled and the user attempts to reveal a plaintext password
in chrome://settings/passwords, they will be prompted to reauthenticate with
their OS password. This matches Safari's behaviour on OS X.

BUG=303113

Review URL: https://codereview.chromium.org/28713002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230479 0039d316-1c4b-4281-b951-d872f2087c98
14 files changed