Don't require getPropertyCSSValue in svg number animation tests

As getPropertyCSSValue is no longer web-exposed we shouldn't require it
in tests so we can delete the interface.

BUG=428595

Review URL: https://codereview.chromium.org/705373004

git-svn-id: svn://svn.chromium.org/blink/trunk@185010 bbb929c8-8fbe-4397-9dbb-9b2b20218538