MD Settings: label and describe a bunch of subpage/open-in-new buttons

chrome/browser/resources/settings/about_page/about_page.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="about_page_browser_proxy.html">
 <link rel="import" href="../i18n_setup.html">
 <link rel="import" href="../icons.html">
 <link rel="import" href="../lifetime_browser_proxy.html">
 <link rel="import" href="../route.html">
 <link rel="import" href="../settings_page/main_page_behavior.html">
 <link rel="import" href="../settings_page/settings_animated_pages.html">
 <link rel="import" href="../settings_page/settings_section.html">
 <link rel="import" href="../settings_page_css.html">
@@ skipping 136 matching lines @@
                 <div class="start">
                   [[promoteUpdaterStatus_.text]]
                   <a href="https://support.google.com/chrome/answer/95414"
                       target="_blank" id="updaterLearnMore"
                       on-tap="onLearnMoreTap_">
                     $i18n{learnMore}
                   </a>
                 </div>
                 <button class="subpage-arrow" is="paper-icon-button-light"
                     disabled="[[promoteUpdaterStatus_.disabled]]"
-                    hidden="[[!promoteUpdaterStatus_.actionable]]">
+                    hidden="[[!promoteUpdaterStatus_.actionable]]"
+                    aria-label="[[promoteUpdaterStatus_.text]]">
                 </button>
               </div>
             </template>
 </if>
             <div id="help" class="settings-box" on-tap="onHelpTap_" actionable>
               <div class="start">$i18n{aboutGetHelpUsingChrome}</div>
-              <button class="icon-external" is="paper-icon-button-light">
-              </button>
+              <button class="icon-external" is="paper-icon-button-light"
+                  aria-labelledby="help"></button>
             </div>
 <if expr="_google_chrome">
             <div id="reportIssue" class="settings-box" actionable
                 on-tap="onReportIssueTap_">
               <div class="start">$i18n{aboutReportAnIssue}</div>
-              <button class="subpage-arrow" is="paper-icon-button-light">
-              </button>
+              <button class="subpage-arrow" is="paper-icon-button-light"
+                  aria-labelledby="reportIssue"></button>
             </div>
 </if>
 <if expr="chromeos">
             <div class="settings-box" on-tap="onDetailedBuildInfoTap_"
                 actionable>
               <div class="start">$i18n{aboutDetailedBuildInfo}</div>
-              <button class="subpage-arrow" is="paper-icon-button-light">
+              <button class="subpage-arrow" is="paper-icon-button-light"
+                  aria-label="$i18n{aboutDetailedBuildInfo}">
               </button>
             </div>
 </if>
             <div class="settings-box product-info copyable">
               <div class="info-section">
                 <div class="secondary">$i18n{aboutProductTitle}</div>
                 <div class="secondary">$i18n{aboutProductCopyright}</div>
               </div>
 
               <div class="info-section">
@@ skipping 19 matching lines @@
               <settings-detailed-build-info></settings-detailed-build-info>
             </settings-subpage>
           </template>
 </if>
         </settings-animated-pages>
       </settings-section>
     </div>
   </template>
   <script src="about_page.js"></script>
 </dom-module>