Oilpan: Make a couple of ScriptWrappable SVG objects garbage collected.

R=haraken@chromium.org,sigbjornf@opera.com,oilpan-reviews@chromium.org

Review URL: https://codereview.chromium.org/526313003

git-svn-id: svn://svn.chromium.org/blink/trunk@181232 bbb929c8-8fbe-4397-9dbb-9b2b20218538