[MD Bookmarks] Add toasts.

chrome/browser/resources/md_bookmarks/toast.js

+// Copyright 2017 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.
+
+Polymer({
+  is: 'bookmarks-toast',
+
+  properties: {
+    open: {
+      type: Boolean,
+      reflectToAttribute: true,
+      observer: 'openChanged_',
+    },
+
+    // Leaving duration undefined or set to 0 will make the toast show forever.
+    duration: Number,

[dpapad, 2017/06/01 17:35:44]

Nit: Can we give this an initial value of 0? This way we don't assign any
semantic meaning to "undefined".

[calamity, 2017/06/02 05:34:22]

Done.

+  },
+
+  /** @private */
+  openChanged_: function() {
+    if (!this.open || !this.duration)
+      return;
+
+    setTimeout(function() {

[dpapad, 2017/06/01 17:35:44]

Shouldn't we be canceling this timeout to avoid race conditions? Example

toast.open = true -> triggers setTimeout()
toast.open = false -> openChanged_() returns early, but previous timeout keeps
going.
toast.open = true -> triggers 2nd setTimeout(). The toast will be closed when
1st timeout expires though, not when the 2nd timeout expires.

[calamity, 2017/06/02 05:34:22]

Good catch!

Done.

+      this.open = false;
+    }.bind(this), this.duration);
+  },
+});