Introduce SVGPropertyHelper to dedupe very similar implementations.

This CL introduces SVGPropertyHelper to dedupe:
- SVGPropertyBase c-tor calls where its argument is always classType()
- |SVGPropertyBase::cloneForAnimation()| implementations

BUG=378670

Review URL: https://codereview.chromium.org/343193011

git-svn-id: svn://svn.chromium.org/blink/trunk@176897 bbb929c8-8fbe-4397-9dbb-9b2b20218538