Check that contain strict and content compute to the longhands (#26201)

They serialize back to the shorthand thanks to the shortest
serialization principle

Author: frivoal

css/css-contain/parsing/contain-computed.html

@@ -24,6 +24,8 @@
 test_computed_value("contain", "style paint");
 test_computed_value("contain", "layout style paint");
 test_computed_value("contain", "size layout style paint");
+test_computed_value("contain", "size layout paint", "strict");
+test_computed_value("contain", "layout paint", "content");
 script>
 body>
 html>