Remove TreatNullAs=NullString for HTMLHeadingElement

http://whatwg.org/html#HTMLHeadingElement-partial

The updated test passes in Firefox Nightly and Opera 12.16,
but throws an exception in IE.

BUG=310298

Review URL: https://codereview.chromium.org/101193003

git-svn-id: svn://svn.chromium.org/blink/trunk@163083 bbb929c8-8fbe-4397-9dbb-9b2b20218538