Fix GetLastUsedProfile to return off the record profile in cros guest mode

BUG=343837
TEST=see bug

Review URL: https://codereview.chromium.org/170113003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252406 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252406