Fix captive portal window on sign-in screen
The captive portal window was being blocked by the GAIA auth extensions's
background script because the script could not tell it from the GAIA login
frame. This CL ensures that the login UI has a unique tab ID, making it
distinguishable from the captive portal window.
BUG=367713
TEST=Manual; updated tests forthcoming in bug 367731
Review URL: https://codereview.chromium.org/
259663006
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@266788 0039d316-1c4b-4281-b951-d872f2087c98