Move debug functions to Window IDL.

This is a cleaner way to expose those functions, and makes it possible to
remove a significant amount code in rust-mozjs.

The assert() function is no longer exposed, as it was unused and not very
useful.

src/components/script/dom/webidls/Window.webidl

@@ -84,3 +84,9 @@ interface WindowTimers {
   void clearInterval(optional long handle = 0);*/
 };
 Window implements WindowTimers;
+
+// Proprietary extensions.
+partial interface Window {
+  void debug(DOMString arg);
+  void gc();
+};