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

git-svn-id: svn://svn.chromium.org/blink/trunk@158014 bbb929c8-8fbe-4397-9dbb-9b2b20218538