Initialize SVGStaticStringLists with correct animated type.

Until now, SVGStaticStringLists were initialized with
AnimatedStringList animated type. This was the reason why SVG
test attributes - with SVGStaticStringList type - were considered
as valid animatable attributes and why we ran into a crash later.
This patch initializes SVGStaticStringLists with AnimatedUnknown
type to avoid any attempts to animate them later.

R=fs@opera.com,kouhei@chromium.org
BUG=427866

Review URL: https://codereview.chromium.org/684833002

git-svn-id: svn://svn.chromium.org/blink/trunk@184567 bbb929c8-8fbe-4397-9dbb-9b2b20218538