Add multiple item selection to BMM on mac

chrome/browser/resources/bookmark_manager/js/cr/ui/listselectionmodel.js

 // Copyright (c) 2010 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.
 
 cr.define('cr.ui', function() {
   const Event = cr.Event;
   const EventTarget = cr.EventTarget;
 
   /**
    * Creates a new selection model that is to be used with lists. This is
@@ skipping 95 matching lines @@
      */
     handleMouseDownUp: function(e, item) {
       var anchorItem = this.anchorItem;
 
       this.beginChange_();
 
       if (!item && !e.ctrlKey && !e.shiftKey && !e.metaKey) {
         this.clear();
       } else {
         var isDown = e.type == 'mousedown';
-        if (!cr.isMac && e.ctrlKey) {
-          // Handle ctrlKey on mouseup
-          if (!isDown) {
+        if (cr.isMac ? e.metaKey : e.ctrlKey) {
+          // Selection is handled at mouseUp on windows/linux, mouseDown on mac.
+          if (cr.isMac? isDown : !isDown) {
             // toggle the current one and make it anchor item
             this.setItemSelected(item, !this.getItemSelected(item));
             this.leadItem = item;
             this.anchorItem = item;
           }
         } else if (e.shiftKey && anchorItem && anchorItem != item) {
           // Shift is done in mousedown
           if (isDown) {
             this.clearAllSelected_();
             this.leadItem = item;
@@ skipping 304 matching lines @@
    * The leadItem is used with multiple selection and it is the item that the
    * user is moving uysing the arrow keys.
    * @type {*}
    */
   cr.defineProperty(ListSelectionModel, 'leadItem', cr.PropertyKind.JS, null);
 
   return {
     ListSelectionModel: ListSelectionModel
   };
 });