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

chrome/browser/resources/settings/device_page/keyboard.html

 <link rel="import" href="chrome://resources/cr_elements/cr_slider/cr_slider.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/paper-icon-button/paper-icon-button-light.html">
 <link rel="import" href="../controls/settings_dropdown_menu.html">
 <link rel="import" href="../controls/settings_toggle_button.html">
 <link rel="import" href="../i18n_setup.html">
 <link rel="import" href="../route.html">
 <link rel="import" href="../settings_shared_css.html">
@@ skipping 91 matching lines @@
                 !prefs.settings.language.xkb_auto_repeat_enabled_r2.value]]"
             aria-labelledby="repeatRateLabel"
             label-min="$i18n{keyRepeatRateSlow}"
             label-max="$i18n{keyRepeatRateFast}">
         </cr-slider>
       </div>
     </iron-collapse>
     <div id="keyboardOverlay" class="settings-box"
         on-tap="onShowKeyboardShortcutsOverlayTap_" actionable>
       <div class="start">$i18n{showKeyboardShortcutsOverlay}</div>
-      <button class="icon-external" is="paper-icon-button-light"></button>
+      <button class="icon-external" is="paper-icon-button-light"
+          aria-label="$i18n{showKeyboardShortcutsOverlay}"></button>
     </div>
     <div class="settings-box" on-tap="onShowLanguageInputTap_" actionable>
       <div class="start">$i18n{keyboardShowLanguageAndInput}</div>
-      <button class="subpage-arrow" is="paper-icon-button-light"></button>
+      <button class="subpage-arrow" is="paper-icon-button-light"
+          aria-label="$i18n{keyboardShowLanguageAndInput}"></button>
     </div>
   </template>
   <script src="keyboard.js"></script>
 </dom-module>