Add a button to launch connectivity diagnostics tool from kiosk mode

BUG=333047

Review URL: https://codereview.chromium.org/149123007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249520 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249520