flimflam: Change the default portal URL from www.google.com to clients3.google.com

Set the default portal URL to clients3.google.com now that we've
registered to use that hostname.  It is preferable to use something
other than www.google.com for machine generated requests.

BUG=chromium-os:18416
TEST=Make sure WiFi, Ethernet, and Mobile Data all move to the online state

Change-Id: I35e63fbf2e5d8285f7004efd65dbbdf44c1d75a0
Reviewed-on: http://gerrit.chromium.org/gerrit/10671
Reviewed-by: Paul Stewart <pstew@chromium.org>
Reviewed-by: Chris Masone <cmasone@chromium.org>
Reviewed-by: Sam Leffler <sleffler@chromium.org>
Tested-by: Jason Glasgow <jglasgow@chromium.org>
Commit-Ready: Jason Glasgow <jglasgow@chromium.org>