Get rid of {Enable|Disable|ChromeCaptivePortalDetector flag.

BUG=316597
TEST=none

Review URL: https://codereview.chromium.org/66713002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233944 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233944