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

chrome/browser/resources/settings/internet_page/internet_page.html

 <link rel="import" href="chrome://resources/cr_elements/icons.html">
 <link rel="import" href="chrome://resources/cr_elements/network/cr_onc_types.html">
 <link rel="import" href="chrome://resources/html/i18n_behavior.html">
 <link rel="import" href="chrome://resources/html/polymer.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="/route.html">
 <link rel="import" href="/settings_page/settings_animated_pages.html">
 <link rel="import" href="/settings_page/settings_subpage.html">
 <link rel="import" href="/settings_shared_css.html">
@@ skipping 20 matching lines @@
             default-network="{{defaultNetwork}}"
             on-show-known-networks="onShowKnownNetworks_"
             networking-private="[[networkingPrivate]]">
         </network-summary>
         <div actionable class="settings-box two-line"
             on-tap="onExpandAddConnectionsTap_">
           <div class="start layout horizontal center">
             <iron-icon icon="cr:add"></iron-icon>
             <div>$i18n{internetAddConnection}</div>
           </div>
-          <cr-expand-button
+          <cr-expand-button alt="$i18n{internetAddConnectionExpandA11yLabel}"
               id="expandAddConnections" expanded="{{addConnectionExpanded_}}">
           </cr-expand-button>
         </div>
         <template is="dom-if" if="[[addConnectionExpanded_]]">
           <div actionable class="settings-box continuation center"
               on-tap="onAddWiFiTap_">
             <div class="start add-no-icon">$i18n{internetAddWiFi}</div>
           </div>
           <div actionable class="settings-box continuation center"
               on-tap="onAddVPNTap_">
@@ skipping 23 matching lines @@
               network-type="[[knownNetworksType_]]"
               on-show-detail="onShowDetail_"
               networking-private="[[networkingPrivate]]">
           </settings-internet-known-networks-page>
         </settings-subpage>
       </template>
     </settings-animated-pages>
   </template>
   <script src="internet_page.js"></script>
 </dom-module>