getComputedStyle returns something even if grid is disabled

The issue was that individual non-stable properties had to check their
runtime flags in CSSComputedStyleDeclaration or else the initial value
would be leaked to the page. Interestingly, this was already tested by
nonstable-css-properties.html which was failing under
--stable-release-mode as we returned a value even if the properties
were disabled.

This change fixes the issue for good for all non-stable properties!
It also fixes nonstable-css-properties.html to avoid passing unneeded
and confusing arguments to getPropertyValue.

BUG=271525

Review URL: https://chromiumcodereview.appspot.com/23536041

git-svn-id: svn://svn.chromium.org/blink/trunk@157478 bbb929c8-8fbe-4397-9dbb-9b2b20218538