Convert FontFaceSource to a mixin

Following whatwg/webidl#433.

Author: foolip

css-font-loading-3/Overview.bs

@@ -990,20 +990,18 @@ Interaction with CSS Font Loading and Matching</h3>
 -->
 
 <h2 id='font-face-source'>
-The <code>FontFaceSource</code> interface</h2>
+The <code>FontFaceSource</code> mixin</h2>
 
 	<pre class='idl'>
-		[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;
 	</pre>
 
-	Any document, workers, or other context which can use fonts in some manner must implement the {{FontFaceSource}} interface.
+	Any document, workers, or other context which can use fonts in some manner must include the {{FontFaceSource}} mixin.
 	The value of the context’s {{fonts}} attribute is its <dfn>font source</dfn>,
 	which provides all of the fonts used in font-related operations,
 	unless defined otherwise.