DNT should return 'null' if there is no client.

BUG=347020
TEST=https://codereview.chromium.org/208243016/

Review URL: https://codereview.chromium.org/208333018

git-svn-id: svn://svn.chromium.org/blink/trunk@170046 bbb929c8-8fbe-4397-9dbb-9b2b20218538