Change WPT test for offset-path parsing

1) As per https://drafts.csswg.org/css-box-4/#typedef-coord-box
<coord-box> can't be defined with margin-box.

2) "at <position" changes come from
w3c/csswg-drafts#8695 (comment)

Resolved here: web-platform-tests/interop#340

Change-Id: I6fe865d5248c7004257cd17669353d810f6e3d09

css/motion/parsing/offset-path-parsing-valid.html

@@ -44,10 +44,12 @@
 
 test_valid_value("offset-path", 'url("http://www.example.com/index.html#polyline1")');
 
-test_valid_value("offset-path", "circle(100px)", "circle(100px at 50% 50%)");
-test_valid_value("offset-path", "margin-box");
+test_valid_value("offset-path", "circle(100px)");
+test_valid_value("offset-path", "circle(100px at 50% 50%)");
+test_valid_value("offset-path", "border-box");
 test_valid_value("offset-path", "inset(10% 20% 30% 40%) border-box");
 test_valid_value("offset-path", "fill-box ellipse(50% 60%)", "ellipse(50% 60%) fill-box");
+test_valid_value("offset-path", "content-box ellipse(50% 60% at 50% 50%)", "ellipse(50% 60% at 50% 50%) content-box");
 </script>
 </body>
 </html>