Reduce rootElement access boilerplate in SVGImage

Add a helper function 'svgRootElement' and use that where appropriate.

Review URL: https://codereview.chromium.org/208503002

git-svn-id: svn://svn.chromium.org/blink/trunk@169751 bbb929c8-8fbe-4397-9dbb-9b2b20218538