[SVG2] Make ownerSVGElement and viewportElement nullable.

Spec: https://svgwg.org/svg2-draft/types.html#InterfaceSVGElement

BUG=366542

Review URL: https://codereview.chromium.org/303483005

git-svn-id: svn://svn.chromium.org/blink/trunk@174809 bbb929c8-8fbe-4397-9dbb-9b2b20218538