Do not paste clipboard contents when middle click on a bookmark in Bookmark Manager.

chrome/browser/resources/bookmark_manager/js/bmm/bookmark_list.js

 // Copyright (c) 2012 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.
 
 // TODO(arv): Now that this is driven by a data model, implement a data model
 //            that handles the loading and the events from the bookmark backend.
 
 cr.define('bmm', function() {
   var List = cr.ui.List;
   var ListItem = cr.ui.ListItem;
@@ skipping 215 matching lines @@
       this.removeEventListener('mouseup', this.handleMiddleMouseUp_);
       if (e.button == 1) {
         var el = e.target;
         while (el.parentNode != this) {
           el = el.parentNode;
         }
         var node = el.bookmarkNode;
         if (node && !bmm.isFolder(node))
           this.dispatchUrlClickedEvent_(node.url, e);
       }
+      e.preventDefault();
     },
 
     // Bookmark model update callbacks
     handleBookmarkChanged: function(id, changeInfo) {
       var dataModel = this.dataModel;
       var index = dataModel.findIndexById(id);
       if (index != -1) {
         var bookmarkNode = this.dataModel.item(index);
         bookmarkNode.title = changeInfo.title;
         if ('url' in changeInfo)
@@ skipping 350 matching lines @@
         }
       }
     }
   };
 
   return {
     BookmarkList: BookmarkList,
     list: list
   };
 });