Revert 158191 "Revert 158014 "Hold onto contextElement() from SV..."

Unable to revert r157949 due to conflicts. So, reverting this fix.

> 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

TBR=inferno@chromium.org

Review URL: https://codereview.chromium.org/23795006

git-svn-id: svn://svn.chromium.org/blink/trunk@158192 bbb929c8-8fbe-4397-9dbb-9b2b20218538