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

chrome/browser/resources/settings/bluetooth_page/bluetooth_page.html

 <link rel="import" href="chrome://resources/cr_elements/cr_scrollable_behavior.html">
 <link rel="import" href="chrome://resources/cr_elements/policy/cr_policy_pref_indicator.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-collapse/iron-collapse.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-flex-layout/classes/iron-flex-layout.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-list/iron-list.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-button/paper-button.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-toggle-button/paper-toggle-button.html">
@@ skipping 30 matching lines @@
         width: 500px;
       }
     </style>
     <settings-animated-pages id="pages" section="bluetooth">
       <neon-animatable route-path="default">
         <div class="settings-box first">
           <div class="layout horizontal center flex">
             <iron-icon icon="settings:bluetooth"></iron-icon>
             <span class="flex">$i18n{bluetoothEnable}</span>
             <cr-expand-button id="expandListButton"
+                alt="$i18n{bluetoothExpandA11yLabel}"
                 hidden$="[[!bluetoothEnabled]]"
                 expanded="{{deviceListExpanded}}">
             </cr-expand-button>
             <paper-toggle-button id="enableBluetooth"
                 checked="{{bluetoothEnabled}}"
                 disabled="[[!adapterState.available]]"
                 on-change="onBluetoothEnabledChange_">
             </paper-toggle-button>
             <cr-policy-pref-indicator
                 pref="[[prefs.cros.device.allow_bluetooth]]"
@@ skipping 40 matching lines @@
           on-device-event="onDeviceEvent_"
           on-response="onResponse_"
           pairing-device="[[pairingDevice]]"
           pairing-event="[[pairingEvent]]">
       </bluetooth-device-dialog>
     </template>
 
   </template>
   <script src="bluetooth_page.js"></script>
 </dom-module>