Dispatch middle click manually by tracking mouse

chrome/browser/resources/ntp4/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);
+
+})();