SVG: Moving animating <svg> to other iframe should not crash.

Moving SVGSVGElement with its SMILTimeContainer already started caused crash before this patch.
|SVGDocumentExtentions::startAnimations()| calls begin() against all SMILTimeContainers in the document, but the SMILTimeContainer for <svg> moved from other document may be already started.

BUG=369860

Review URL: https://codereview.chromium.org/290353002

git-svn-id: svn://svn.chromium.org/blink/trunk@174338 bbb929c8-8fbe-4397-9dbb-9b2b20218538