Update do not track IDL to correctly return null when a null string is returned
Update do not track IDL to correctly return null when a null string is returned

BUG=230363
TEST=not returning null breaks the browser_test DoNotTrackTest.DOMProperty

Review URL: https://codereview.chromium.org/13932004

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@148263 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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