Use ScriptWrappable::init less

Use ScriptWrappable::init less, it is only needed for most derived classes usually, but these SVG classes are not exposed as JS type.

Review URL: https://codereview.chromium.org/328703005

git-svn-id: svn://svn.chromium.org/blink/trunk@176073 bbb929c8-8fbe-4397-9dbb-9b2b20218538