cros: A LoginUI for kiosk app identity api sign-in.
- Move gaia auth extension loading from ComponentLoader to a
ScopedGaiaAuthExtension class;
- Use the ScopedGaiaAuthExtension to load gaia auth extension when
it is needed for login screen and app login dialog;
- Use GaiaAuthHost to create an app login webui dialog;
- Wire the dialog in LoginUIService::ShowLoginPopup for kiosk app;
- Fix gaia auth margin/padding so that the input fields are horizontally
centered in the dialog (and no regression on login screen);
BUG=224336
Review URL: https://chromiumcodereview.appspot.com/
14328016
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@196656 0039d316-1c4b-4281-b951-d872f2087c98