web-sys: Use mixins instead of [NoInterfaceObject] interfaces and implements

I think these might all be from before WebIDL mixins existed. Either way,
multiple inheritance of interfaces that don't have exposed interface objects is
equivalent to mixins.

crates/web-sys/webidls/enabled/FrameLoader.webidl

@@ -209,12 +209,11 @@ interface FrameLoader {
  *        The nsIWebBrowserPersistDocumentReceiver is a callback that
  *        will be fired once the document is ready for persisting.
  */
-[NoInterfaceObject]
-interface WebBrowserPersistable
+interface mixin WebBrowserPersistable
 {
   [Throws]
   void startPersistence(unsigned long long aOuterWindowID,
                         nsIWebBrowserPersistDocumentReceiver aRecv);
 };
 
-FrameLoader implements WebBrowserPersistable;
+FrameLoader includes WebBrowserPersistable;