[css-animations] Change definition of GlobalEventHandlers to mixin (#2797)

GlobalEventHandlers is now defined as a mixin per https://html.spec.whatwg.org/multipage/webappapis.html#globaleventhandlers. Update the partial interface's definition to indicate that it is a mixin.

Author: csnardi

css-animations-1/Overview.bs

@@ -1308,7 +1308,7 @@ events</a> as defined in
 IDL Definition</h4>
 
 <pre class="idl">
-partial interface GlobalEventHandlers {
+partial interface mixin GlobalEventHandlers {
   attribute EventHandler onanimationstart;
   attribute EventHandler onanimationiteration;
   attribute EventHandler onanimationend;