Remove TreatNullAs=NullString for SVGElement.xmlbase

http://www.w3.org/TR/SVG11/types.html#InterfaceSVGElement

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

BUG=310298

Review URL: https://codereview.chromium.org/138433003

git-svn-id: svn://svn.chromium.org/blink/trunk@165083 bbb929c8-8fbe-4397-9dbb-9b2b20218538