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
HashSet<SVGElement*>* SVGDocumentExtensions::setOfElementsReferencingTarget(SVGElement* referencedElement) const
{
    ASSERT(referencedElement);
    const HashMap<SVGElement*, OwnPtr<HashSet<SVGElement*> > >::const_iterator it = m_elementDependencies.find(referencedElement);
    if (it == m_elementDependencies.end())
        return 0;
    return it->value.get();
}

HashSet<SVGElement*>* SVGDocumentExtensions::setOfElementsReferencingTarget(SVGElement* referencedElement) const
{
    ASSERT(referencedElement);
    const HashMap<SVGElement*, OwnPtr<HashSet<SVGElement*> > >::const_iterator it = m_elementDependencies.find(referencedElement);
    if (it == m_elementDependencies.end())
        return 0;
    return it->value.get();
}
