MD Settings: Fix rendering of popup menus inside iron-lists.

chrome/browser/resources/settings/list_item_behavior.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.
+
+/** @polymerBehavior */
+var ListItemBehavior = {
+  ensureMaxZIndex: function() {
+    // <iron-list> applies translate3d() transform on every element, which
+    // means that in the natural order the (n-1)-th element is at a lower z
+    // index than the n-th element. This prevents the popup menu from being
+    // painted on top of other elements, and therefore a zIndex adjustment is
+    // necessary.
+    this.style.zIndex = ++ListItemBehavior.maxZIndex_;
+  },
+};
+
+/**
+ * The max z index assigned to an <iron-list> item so far. Used to
+ * ensure that the per-item popup menu is displayed correctly.
+ * @private {number}
+ */
+ListItemBehavior.maxZIndex_ = 0;