Remove remaining testing of getPropertyCSSValue

As getPropertyCSSValue, CSSPrimitiveValue, etc. are no longer
web-exposed, we should stop testing them so we can delete the interface.

BUG=428595

Review URL: https://codereview.chromium.org/715913003

git-svn-id: svn://svn.chromium.org/blink/trunk@185165 bbb929c8-8fbe-4397-9dbb-9b2b20218538