Remove TreatNullAs=NullString for SVGScriptElement.type

http://www.w3.org/TR/SVG/script.html#InterfaceSVGScriptElement

The test passes in Firefox Nightly and IE11 Release Preview, but fails
in Opera 12.16.

BUG=310298

Review URL: https://codereview.chromium.org/130503013

git-svn-id: svn://svn.chromium.org/blink/trunk@165366 bbb929c8-8fbe-4397-9dbb-9b2b20218538