Improve test coverage of TreatNullAs=NullString and TreatUndefinedAs=NullString

BUG=334519

Review URL: https://codereview.chromium.org/138483004

git-svn-id: svn://svn.chromium.org/blink/trunk@165178 bbb929c8-8fbe-4397-9dbb-9b2b20218538