Add two more options for the --password-store switch

Currently the option "gnome" is ambiguous between Keyring and Libsecret.
The two new options, "gnome-keyring" and "gnome-libsecret" allow
targeting those implementations individually.

Since password manager and OSCrypt both use this switch to select their
backends, the selection logic has been turned into a common util, to
avoid code duplication.

BUG=602624

Review-Url: https://codereview.chromium.org/2256633002
Cr-Commit-Position: refs/heads/master@{#412540}
6 files changed