Make css-properties-position-relative-as-parent-fixed.html a ref-test

Convert fast/css/css-properties-position-relative-as-parent-fixed.html
in a ref-test test avoiding the need to rebaseline the expected results
if we do any modification.

BUG=31286,425991
TEST=fast/css/css-properties-position-relative-as-parent-fixed.html

Review URL: https://codereview.chromium.org/669143002

git-svn-id: svn://svn.chromium.org/blink/trunk@185219 bbb929c8-8fbe-4397-9dbb-9b2b20218538