Restrict fetchLater() to secure contexts

Already tested in https://github.com/web-platform-tests/wpt/blob/c009ca2fa7d311b1a257222aa0f9fd7706ffa4d6/fetch/fetch-later/non-secure.window.js and logically correct given permissions are involved.

Author: TimvdLippe

fetch.bs

@@ -9405,7 +9405,7 @@ interface FetchLaterResult {
 };
 
 partial interface Window {
-  [NewObject] FetchLaterResult fetchLater(RequestInfo input, optional DeferredRequestInit init = {});
+  [NewObject, SecureContext] FetchLaterResult fetchLater(RequestInfo input, optional DeferredRequestInit init = {});
 };
 </pre>