Introduce a button to toggle hiding/showing of the details in the events tab.

chrome/browser/resources/sync_internals/events.html

 <style>
 #sync-events-table,
 #sync-events-table th,
 #sync-events-table td {
   border: 1px black solid;
 }
 
 #sync-events-table {
   width: 100%;
 }
 
 #sync-events > tr {
   vertical-align: top;
 }
+
+.hidden {
+  display: none;
+}
 </style>
 
 <table id="sync-events-table">
   <thead>
     <th>Time</th>
     <th>Submodule</th>
     <th>Event</th>
     <th>Details</th>
   </thead>
   <tbody id="sync-events">
   </tbody>
 </table>
 
 <script src="chrome://sync-internals/events.js"></script>