Bug 1541098 [wpt PR 16128] - Update interfaces/css-font-loading.idl, a=testonly

Automatic update from web-platform-tests
Update interfaces/css-font-loading.idl (#16128)

Source: https://github.com/tidoust/reffy-reports/blob/e077277/whatwg/idl/css-font-loading.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/512363424
--

wpt-commits: 588567872385afc2864087163ec9624262d55095
wpt-pr: 16128

testing/web-platform/tests/interfaces/css-font-loading.idl

@@ -81,11 +81,9 @@ interface FontFaceSet : EventTarget {
   readonly attribute FontFaceSetLoadStatus status;
 };
 
-[Exposed=(Window,Worker),
- NoInterfaceObject]
-interface FontFaceSource {
+interface mixin FontFaceSource {
   readonly attribute FontFaceSet fonts;
 };
 
-Document implements FontFaceSource;
-WorkerGlobalScope implements FontFaceSource;
+Document includes FontFaceSource;
+WorkerGlobalScope includes FontFaceSource;