Prepare SVGDocumentExtensions for oilpan

This patch removes all RefPtr<SVGSVGElement>.

BUG=357163

Review URL: https://codereview.chromium.org/280263002

git-svn-id: svn://svn.chromium.org/blink/trunk@173941 bbb929c8-8fbe-4397-9dbb-9b2b20218538