MD Webui: Improve accessibility of toolbar search field

ui/webui/resources/cr_elements/cr_toolbar/cr_toolbar_search_field.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-search-field',
 
   behaviors: [CrSearchFieldBehavior],
 
   properties: {
@@ skipping 63 matching lines @@
   /**
    * @param {boolean} narrow
    * @return {number}
    * @private
    */
   computeIconTabIndex_: function(narrow) {
     return narrow ? 0 : -1;
   },
 
   /**
+   * @param {boolean} narrow
+   * @return {string}
+   * @private
+   */
+  computeIconAriaHidden_: function(narrow) {
+    return Boolean(!narrow).toString();
+  },
+
+  /**
    * @return {boolean}
    * @private
    */
   computeIsSpinnerShown_: function() {
     return this.spinnerActive && this.showingSearch;
   },
 
   /** @private */
   onInputFocus_: function() {
     this.searchFocused_ = true;
@@ skipping 42 matching lines @@
 
     if (this.showingSearch) {
       this.focus_();
       return;
     }
 
     this.setValue('');
     this.getSearchInput().blur();
   },
 });