[SVG] className tear-off should be initialized after m_isContextElement flag.

Before this patch, |SVGElement::m_isContextElement| was re-initialized to "false" after it is set to true by NewSVGAnimatedPropertyBase c-tor called in m_className.
This patch changes |m_className| tear-off instantiation after |m_isContextElement| flag is initialized.

BUG=340048

Review URL: https://codereview.chromium.org/152343004

git-svn-id: svn://svn.chromium.org/blink/trunk@166298 bbb929c8-8fbe-4397-9dbb-9b2b20218538