Don't require getPropertyCSSValue in animation fill-mode tests

As getPropertyCSSValue is no longer web-exposed, we should stop testing
it so we can delete the interface. In animation fill-mode tests, it
isn't needed as we can just call parseFloat(style.left), as we always
return values in the format "123px" and parseFloat drops trailing
characters.

BUG=428595

Review URL: https://codereview.chromium.org/707713005

git-svn-id: svn://svn.chromium.org/blink/trunk@184976 bbb929c8-8fbe-4397-9dbb-9b2b20218538