Fixed detector behaviour for online -> offline transitions.

BUG=307033
TEST=NetworkPortalDetectorImplTest.*

Review URL: https://codereview.chromium.org/94373002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237780 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237780