Revert middle click related changes

chrome/browser/resources/synthetic_middleclick.js

-// Copyright 2016 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.
-
-(function() {
-
-var middleButtonMouseDownTarget = null;
-
-document.addEventListener('mousedown', function(e) {
-  if (e.button == 1)
-    middleButtonMouseDownTarget = e.path[0];
-}, true);
-
-document.addEventListener('mouseup', function(e) {
-  if (e.button != 1)
-    return;
-
-  if (e.path.length > 0 && e.path[0] == middleButtonMouseDownTarget)
-    middleButtonMouseDownTarget.dispatchEvent(new MouseEvent('click', e));
-
-  middleButtonMouseDownTarget = null;
-}, true);
-
-window.addEventListener('click', function(e) {
-  if (e.button == 1 && !e.isTrusted)
-    e.preventDefault();
-});
-
-})();