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/webidls/TestBindingProxy.webidl

@@ -13,6 +13,7 @@
 // web pages.
 
 interface TestBindingProxy : TestBinding {
+  readonly attribute unsigned long length;
 
   getter DOMString getNamedItem(DOMString name);