MD History: Add tooltip for history menu button

ui/webui/resources/cr_elements/cr_toolbar/cr_toolbar.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.
 
 Polymer({
   is: 'cr-toolbar',
 
   properties: {
     // Name to display in the toolbar, in titlecase.
     pageName: String,
 
     // Prompt text to display in the search field.
     searchPrompt: String,
 
     // Tooltip to display on the clear search button.
     clearLabel: String,
 
+    // Tooltip to display on the menu button.
+    buttonLabel: String,

[tsergeant, 2016/08/15 03:02:47]

How about calling this `menuLabel`? buttonLabel could apply to any button in the
toolbar.

[lshang, 2016/08/15 05:39:10]

Done.

+
     // Value is proxied through to cr-toolbar-search-field. When true,
     // the search field will show a processing spinner.
     spinnerActive: Boolean,
 
     // Controls whether the menu button is shown at the start of the menu.
     showMenu: {
       type: Boolean,
       reflectToAttribute: true,
       value: true
     },
@@ skipping 14 matching lines @@
   /** @return {!CrToolbarSearchFieldElement} */
   getSearchField: function() {
     return this.$.search;
   },
 
   /** @private */
   onMenuTap_: function(e) {
     this.fire('cr-menu-tap');
   }
 });