Continue removing testing of getPropertyCSSValue

As getPropertyCSSValue is no longer web-exposed, we should stop testing
it so we can delete the interface. Instead of using getPropertyCSSValue
and calling cssText on the returned value, we can use getPropertyValue
or the direct accessors.

BUG=428595

Review URL: https://codereview.chromium.org/703323002

git-svn-id: svn://svn.chromium.org/blink/trunk@184988 bbb929c8-8fbe-4397-9dbb-9b2b20218538