kiosk: Don't exit fullscreen on ESC.

BUG=328489

Review URL: https://codereview.chromium.org/101623012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241476 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241476