Fix register/unregisterProperty on CSS interface to be static.

See @tabatkins' comment at [1].

[1]: w3c#216 (comment)

Author: Jonathan Chan

css-properties-values-api/Overview.bs

@@ -68,8 +68,8 @@ dictionary PropertyDescriptor {
 };
 
 partial interface CSS {
-  void registerProperty(PropertyDescriptor descriptor);
-  void unregisterProperty(DOMString name);
+  static void registerProperty(PropertyDescriptor descriptor);
+  static void unregisterProperty(DOMString name);
 };
 </pre>