Don't test CSSValue in http/tests/security tests

As CSSValue is no longer web-exposed there's no point testing it.
Removing testing of it will allow us to delete the interface.

BUG=428595

Review URL: https://codereview.chromium.org/711043004

git-svn-id: svn://svn.chromium.org/blink/trunk@185071 bbb929c8-8fbe-4397-9dbb-9b2b20218538