Add a flag to skip robot token fetching for testing device enrollments.

None of the testing server implementations are currently capable of minting
robot OAuth tokens and testing of device enrollment was only possible against
actual DMServer. With this flag it will be possible to successfully complete
enrollment while still testing the whole enrollment handshake and only skipping
the step where the robot token is fetched from GAIA.

BUG=313811
TEST=Enrollment against YAPS should succeed.
R=joaodasilva@chromium.org

Review URL: https://codereview.chromium.org/60113002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233262 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233262