Reset the device policy machinery upon retrying enrollment.

BUG=chromium-os:18208
TEST=See bug description


Review URL: http://codereview.chromium.org/7676005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@97615 0039d316-1c4b-4281-b951-d872f2087c98
void DeviceTokenFetcher::StopAutoRetry() {
void DeviceTokenFetcher::Reset() {
  SetState(STATE_INACTIVE);
 }

void DeviceTokenFetcher::StopAutoRetry() {
  scheduler_->CancelDelayedWork();
  backend_.reset();
 }
