DNT should return 'null' if there is no client.
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@170046 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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