MD Webui: Improve accessibility of toolbar search field

ui/webui/resources/cr_elements/cr_toolbar/cr_toolbar_search_field.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-icon-button/paper-icon-button.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-spinner/paper-spinner-lite.html">
 <link rel="import" href="chrome://resources/cr_elements/cr_search_field/cr_search_field_behavior.html">
 <link rel="import" href="chrome://resources/cr_elements/icons.html">
 
 <dom-module id="cr-toolbar-search-field">
   <template>
     <style>
       :host {
@@ skipping 116 matching lines @@
       :host([narrow][showing-search]) #icon,
       :host([narrow][showing-search]) paper-spinner-lite {
         -webkit-margin-start: 18px;
       }
     </style>
     <template is="dom-if" if="[[isSpinnerShown_]]">
       <paper-spinner-lite active>
       </paper-spinner-lite>
     </template>
     <paper-icon-button id="icon" icon="cr:search" title="[[label]]"
-        tabindex$="[[computeIconTabIndex_(narrow)]]">
+        tabindex$="[[computeIconTabIndex_(narrow)]]"
+        aria-hidden$="[[computeIconAriaHidden_(narrow)]]">
     </paper-icon-button>
     <div id="searchTerm">
-      <label id="prompt" for="searchInput">[[label]]</label>
+      <label id="prompt" for="searchInput" aria-hidden="true">[[label]]</label>
       <input id="searchInput"
           type="search"
           on-input="onSearchTermInput"
           on-search="onSearchTermSearch"
           on-keydown="onSearchTermKeydown_"
           on-focus="onInputFocus_"
           on-blur="onInputBlur_"
           incremental
           autofocus>
       </input>
     </div>
     <template is="dom-if" if="[[hasSearchText]]">
       <paper-icon-button icon="cr:cancel" id="clearSearch"
           title="[[clearLabel]]" on-tap="clearSearch_">
       </paper-icon-button>
     </template>
   </template>
   <script src="cr_toolbar_search_field.js"></script>
 </dom-module>