Add dfn to performance timeline task source

This enables linking to it from other specifications. In particular this will be used in ResourceTiming for the buffer full task: w3c/resource-timing#215

index.html

@@ -717,8 +717,8 @@ <h2>Queue the PerformanceObserver task</h2>
         </li>
         <li>
           <a>Queue a task</a> that consists of running the following substeps.
-          The <a>task source</a> for the queued task is the <i>performance
-          timeline</i> task source.
+          The <a>task source</a> for the queued task is the <dfn data-export="">performance
+          timeline task source</dfn>.
           <ol>
             <li>Unset <a>performance observer task queued flag</a> of
             <var>relevantGlobal</var>.