Prevent destruction of self in SMILTimeContainer with 'discard'

If discarding the <svg> root an animation update could end up destroying
the SVGSVGElement owning the SMILTimeContainer, leading to use-after-free.
Make sure to keep an additional reference to the owning SVGSVGElement
across affected callsites.

BUG=351316

Review URL: https://codereview.chromium.org/195233003

git-svn-id: svn://svn.chromium.org/blink/trunk@169239 bbb929c8-8fbe-4397-9dbb-9b2b20218538