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

chrome/browser/resources/settings/printing_page/cups_printer_details_page.html

 <link rel="import" href="chrome://resources/cr_elements/cr_expand_button/cr_expand_button.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/paper-input/paper-input.html">
 <link rel="import" href="/settings_shared_css.html">
 
 <dom-module id="settings-cups-printer-details-page">
   <template>
     <style include="settings-shared">
       paper-input {
@@ skipping 18 matching lines @@
       <div class="start">
         <div>$i18n{printerModel}</div>
         <div class="secondary">[[printer.printerModel]]</div>
       </div>
     </div>
     <!-- TODO(xdai): Add printerStatus. -->
 
     <!-- Advanced toggle -->
     <div class="settings-box" actionable on-tap="toggleAdvancedExpanded_">
       <div class="start">$i18n{printerDetailsAdvanced}</div>
-      <cr-expand-button id="expandButton" expanded="{{advancedExpanded}}">
+      <cr-expand-button id="expandButton" expanded="{{advancedExpanded}}"
+          alt="$i18n{printerDetailsA11yLabel}">
       </cr-expand-button>
     </div>
     <!-- Advanced section -->
     <iron-collapse opened="[[advancedExpanded]]">
       <div class="list-frame vertical-list">
         <div class="list-item setting-box two-line">
           <div class="start">
             <div>$i18n{printerAddress}</div>
             <div class="secondary">[[printer.printerAddress]]</div>
           </div>
         </div>
         <div class="list-item setting-box two-line">
           <div class="start">
             <div>$i18n{printerProtocol}</div>
             <div class="secondary">[[printer.printerProtocol]]</div>
           </div>
         </div>
       </div>
     </iron-collapse>
   </template>
   <script src="cups_printer_details_page.js"></script>
 </dom-module>