Revert 158014 "Hold onto contextElement() from SVGAnimated* wrap..."

Needed for reverting r157949 which is still causing ton of use-after-fress on ClusterFuzz.

> Hold onto contextElement() from SVGAnimated* wrappers.
> 
> SVG tearoff types holds onto its contextElement() from r157949, but SVGAnimated tearoffs didn't.
> This patch fixes the issue.
> 
> BUG=294464
> BUG=270000
> 
> Review URL: https://chromiumcodereview.appspot.com/23708048

TBR=kouhei@chromium.org

Review URL: https://codereview.chromium.org/23516016

git-svn-id: svn://svn.chromium.org/blink/trunk@158191 bbb929c8-8fbe-4397-9dbb-9b2b20218538