[MD-Settings A11y] Set alt text for expandable buttons.

chrome/browser/resources/settings/languages_page/languages_page.html

 <link rel="import" href="chrome://resources/html/assert.html">
 <link rel="import" href="chrome://resources/html/cr.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-collapse/iron-collapse.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-icon/iron-icon.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/neon-animation/neon-animatable.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-toggle-button/paper-toggle-button.html">
 <link rel="import" href="chrome://resources/cr_elements/cr_expand_button/cr_expand_button.html">
 <link rel="import" href="chrome://resources/cr_elements/cr_shared_menu/cr_shared_menu.html">
@@ skipping 28 matching lines @@
     <settings-animated-pages id="pages" section="languages">
       <neon-animatable route-path="default">
         <div class="settings-box first two-line">
           <div class="start">
             <div>$i18n{languagesListTitle}</div>
             <div class="secondary">
               [[getProspectiveUILanguageName_(
                     languages, prefs.intl.app_locale.value)]]
             </div>
           </div>
-          <cr-expand-button expanded="{{languagesOpened_}}">
+          <cr-expand-button expanded="{{languagesOpened_}}"
+              alt="$i18n{languagesExpandA11yLabel}">
           </cr-expand-button>
         </div>
         <iron-collapse id="languagesCollapse" opened="[[languagesOpened_]]">
           <div class="list-frame vertical-list">
             <span class="list-item"
                 hidden="[[isHelpTextHidden_(languages.enabled.*)]]">
               <span>$i18n{orderLanguagesInstructions}</span>
               <a href="$i18n{languagesLearnMoreURL}" target="_blank">
                 $i18n{learnMore}
               </a>
@@ skipping 31 matching lines @@
         </iron-collapse>
 <if expr="chromeos">
         <div id="manage-input-methods-subpage-trigger"
               class="settings-box two-line">
           <div class="start">
             <div>$i18n{inputMethodsListTitle}</div>
             <div class="secondary">
               [[getInputMethodName_(languages.inputMethods.currentId)]]
             </div>
           </div>
-          <cr-expand-button expanded="{{inputMethodsOpened_}}">
+          <cr-expand-button expanded="{{inputMethodsOpened_}}"
+              alt="$i18n{inputMethodsExpandA11yLabel}">
           </cr-expand-button>
         </div>
         <iron-collapse id="inputMethodsCollapse"
             opened="[[inputMethodsOpened_]]">
           <div class="list-frame vertical-list">
             <template is="dom-repeat"
                 items="[[languages.inputMethods.enabled]]">
               <div class$="list-item [[getInputMethodItemClass_(
                       item.id, languages.inputMethods.currentId)]]"
                   on-tap="onInputMethodTap_" actionable>
@@ skipping 18 matching lines @@
         </iron-collapse>
 </if>
 <if expr="not is_macosx">
         <div id="spellcheck-subpage-trigger" class="settings-box two-line">
           <div class="start">
             <div>$i18n{spellCheckListTitle}</div>
             <div class="secondary">
               [[getSpellCheckSecondaryText_(languages.enabled.*)]]
             </div>
           </div>
-          <cr-expand-button expanded="{{spellCheckOpened_}}">
+          <cr-expand-button expanded="{{spellCheckOpened_}}"
+              alt="$i18n{spellCheckExpandA11yLabel}">
           </cr-expand-button>
         </div>
         <iron-collapse id="spellCheckCollapse" opened="[[spellCheckOpened_]]">
           <div class="list-frame vertical-list">
             <template is="dom-repeat" items="[[languages.enabled]]">
               <div class="list-item">
                 <label class="start"
                     id="supportsSpellCheckLabel-[[item.language.code]]">
                   <span disabled$="[[!item.language.supportsSpellcheck]]">
                     [[item.language.displayName]]
@@ skipping 70 matching lines @@
 </if>
     </settings-animated-pages>
     <template is="dom-if" if="[[showAddLanguagesDialog_]]" restamp>
       <settings-add-languages-dialog languages="{{languages}}"
           language-helper="[[languageHelper]]">
       </settings-add-languages-dialog>
     </template>
   </template>
   <script src="languages_page.js"></script>
 </dom-module>