[css-font-loading] Return false from delete() with a CSS-connected font-face.

Author: tabatkins

css-font-loading/Overview.bs

@@ -968,7 +968,8 @@ Interaction with CSS’s ''@font-face'' Rule</h3>
 	and throw an {{InvalidModificationError}}.
 
 	When a {{FontFaceSet}} object's {{FontFaceSet/delete()}} method is called with a <a>CSS-connected</a> {{FontFace}} object,
-	the operation must be a no-op.
+	the operation must be a no-op,
+	and return <code class="lang-javascript">false</code>.
 
 	Note: Authors can still maintain references to a removed {{FontFace}},
 	even if it's been automatically removed from a <a>font source</a>.