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Also sniffer listener was implemented on the WebUI side, which signals when
GAIA page loading error appears.

BUG=chromium-os:18744
TEST=Behind captive portal, try to click 'Add new user'. Corresponding message page should be shown.

Review URL: http://codereview.chromium.org/7779010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@99777 0039d316-1c4b-4281-b951-d872f2087c98