cros: Ensure system salt is loaded for enterprise and kiosk.

DeviceOAuth2TokenService needs system salt for encryption and decryption.
The system salt needs to be loaded before enrolling a kiosk device or
launching an enterprise kiosk app.

BUG=305858

Review URL: https://codereview.chromium.org/26694007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228189 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228189
Prevent extension side-enabling.

NOTRY=True

Review URL: https://codereview.chromium.org/26714003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228190 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228190