Remove TreatNullAs=NullString for HTMLFieldSetElement

http://www.whatwg.org/specs/web-apps/current-work/multipage/the-button-element.html#htmlfieldsetelement

The test passes only in Firefox Nightly, since neither IE11 Release
Preview nor Opera 12.16 supports the property at all, verified by
document.createElement('fieldset').name == undefined.

BUG=310298

Review URL: https://codereview.chromium.org/70103011

git-svn-id: svn://svn.chromium.org/blink/trunk@162023 bbb929c8-8fbe-4397-9dbb-9b2b20218538