Allow random parameters of consistent type

Corresponds to w3c/csswg-drafts@f587be0 which updated the criteria for
random arguments to be of consistent rather than same type

Author: Calme1709

css/css-values/random-invalid.tentative.html

@@ -29,6 +29,5 @@
 test_invalid_value('width', 'random(fixed 0.5%, 1px, 2px)');
 test_invalid_value('width', 'random(fixed -1, 1px, 2px)');
 test_invalid_value('width', 'random(10deg, 20deg)');
-test_invalid_value('width', 'random(10px, 20%)');
 
 </script>

css/css-values/random-serialize.tentative.html

@@ -24,6 +24,9 @@
 test_valid_value('width', 'random(element-shared auto, 0px, 100px, 50px)', 'random(element-shared, 0px, 100px, 50px)');
 test_valid_value('width', 'random(fixed 0.5, 0px, 100px, 50px)');
 
+// Test consistent types
+test_valid_value('width', 'random(10px, 20%)');
+
 // Test out of order.
 test_valid_value('width', 'random(100px, 0px)');