Allow calculated values for padding.

This is a follow up to 17a2f01,
which began to enforce the non-negative value restriction on
<padding-width>.

Author: cscott

src/css/ValidationTypes.js

@@ -181,7 +181,8 @@ var ValidationTypes = {
         },
 
         "<padding-width>": function(part){
-            return part.value >= 0 && (this["<length>"](part) || this["<percentage>"](part));
+            return (this["<length>"](part) || this["<percentage>"](part)) &&
+                (String(part) === "0" || part.type === "function" || (part.value) >= 0);
         },
 
         "<shape>": function(part){

tests/css/Validation.js

@@ -1202,8 +1202,11 @@
         property: "padding-left",
 
         valid: [
+            "0",
             "6px",
             "3%",
+            "1em",
+            "calc(100% - 80px)",
             "inherit"
         ],