Initial implementation of ownPropertyKeys proxy handler

Generates `SupportedPropertyNames` on DOM structs that should implement
it. Most of them are unimplemented now (which can be implemented in
later PRs), with the exception of `HTMLCollection`. Also added a couple
relevant WPT tests.

Closes #6390

Closes #2215

components/script/dom/storage.rs

@@ -134,6 +134,11 @@ impl<'a> StorageMethods for &'a Storage {
     fn NamedDeleter(self, name: DOMString) {
         self.RemoveItem(name);
     }
+
+    fn SupportedPropertyNames(self) -> Vec<DOMString> {
+        // FIXME: unimplemented (https://github.com/servo/servo/issues/7273)
+        vec![]
+    }
 }
 
 trait PrivateStorageHelpers {