Remove TreatNullAs=NullString for HTMLMarqueeElement

http://whatwg.org/html#htmlmarqueeelement

Very few of the tests pass in any other browser, with little
agreement between them. There might be spec or implementation
work to do here, but this change is likely safe regardless.

BUG=310298

Review URL: https://codereview.chromium.org/98893003

git-svn-id: svn://svn.chromium.org/blink/trunk@163035 bbb929c8-8fbe-4397-9dbb-9b2b20218538