First step in getting chrome to use a web-based sign in flow. This change

      is behind a flag and does not work without the corresponding changes in Gaia.
I am committing this so that people can start testing this during developement.

BUG=137118
TEST=To enable the web-based flow, use the command line argument
--use-web-based-signin-flow.  To use an alternate/testing Gaia server, also
use the command line argument --gaia-host=<host-name>.  <host-name> should
include only the machine name and an optional port number.


Review URL: https://chromiumcodereview.appspot.com/11092029

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@166913 0039d316-1c4b-4281-b951-d872f2087c98