Use interface mixins instead of [NoInterfaceObject]

WebIDL recently introduced dedicated syntax for mixins[1]. So, we can
replace `[NoInterfaceObject]` and `implements` with `interface mixin` and
`includes`.

This following interface is impacted by this change:
  - NavigatorStorage

This fixes whatwg#53 issue.

[1] whatwg/webidl@45e8173

storage.bs

@@ -221,14 +221,12 @@ user agent should alert the user and offer a way to clear <a>persistent buckets<
 <h2 id=api>API</h2>
 
 <pre class=idl>
-[SecureContext,
- NoInterfaceObject,
- Exposed=(Window,Worker)]
-interface NavigatorStorage {
+[SecureContext]
+interface mixin NavigatorStorage {
   readonly attribute StorageManager storage;
 };
-Navigator implements NavigatorStorage;
-WorkerNavigator implements NavigatorStorage;
+Navigator includes NavigatorStorage;
+WorkerNavigator includes NavigatorStorage;
 </pre>
 
 Each <a>environment settings object</a> has an associated {{StorageManager}} object.