Handle when service no longer exists during mobile activation.

When the cellular service no longer exists during mobile activation, a
NULL |network| is passed to MobileActivator::ChangeState(), which wasn't
handled properly and led to a crash due to NULL pointer dereference.
This CL fixes MobileActivator and MobileSetupUI to properly handle a
NULL |network|.

BUG=325404
TEST=Manually tested 3G activation with a Gobi3k modem.

Review URL: https://codereview.chromium.org/108793002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239833 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239833