feedback UI: use native window controls on Mac

chrome/browser/resources/feedback/js/topbar_handlers.js

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * Setup handlers for the minimize and close topbar buttons.
  */
 function initializeHandlers() {
+  // If this dialog is using system window controls, these elements aren't
+  // needed at all.
+  if (window.feedbackInfo.useSystemWindowFrame) {
+    $('minimize-button').hidden = true;
+    $('close-button').hidden = true;
+    return;
+  }
   $('minimize-button').addEventListener('click', function(e) {
     e.preventDefault();
     chrome.app.window.current().minimize();
   });
 
   $('minimize-button').addEventListener('mousedown', function(e) {
     e.preventDefault();
   });
 
   $('close-button').addEventListener('click', function() {
     window.close();
   });
 
   $('close-button').addEventListener('mousedown', function(e) {
     e.preventDefault();
   });
 }
 
 window.addEventListener('DOMContentLoaded', initializeHandlers);