Don't require getPropertyCSSValue in inline-style-allowed-while-cloning-objects.html

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/713613002

git-svn-id: svn://svn.chromium.org/blink/trunk@184998 bbb929c8-8fbe-4397-9dbb-9b2b20218538