[MD settings] add sub-label to Show Home Button check box

chrome/browser/resources/settings/appearance_page/appearance_page.html

 <link rel="import" href="chrome://resources/polymer/v1_0/polymer/polymer.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-icon/iron-icon.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-radio-button/paper-radio-button.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-toggle-button/paper-toggle-button.html">
 <link rel="import" href="chrome://md-settings/controls/settings_dropdown_menu.html">
 <link rel="import" href="chrome://md-settings/controls/settings_input.html">
 <link rel="import" href="chrome://md-settings/controls/settings_radio_group.html">
@@ skipping 31 matching lines @@
           <template is="dom-if" if="[[!allowResetTheme_]]">
             <iron-icon icon="exit-to-app"></iron-icon>
           </template>
           <template is="dom-if" if="[[allowResetTheme_]]">
             <paper-button on-tap="resetTheme_"
                 >[[i18n('resetToDefaultTheme')]]</paper-button>
           </template>
         </div>
         <div class="settings-box two-line">
           <settings-checkbox class="start" i18n-values="label:showHomeButton"
+              sub-label="[[getShowHomeSubLabel_(prefs.homepage_is_newtabpage.value, prefs.homepage.value)]]"
               pref="{{prefs.browser.show_home_button}}">
           </settings-checkbox>
         </div>
         <template is="dom-if" if="[[prefs.browser.show_home_button.value]]">
           <div class="list-frame">
             <settings-radio-group pref="{{prefs.homepage_is_newtabpage}}">
               <paper-radio-button class="list-item" name="true">
                  [[i18n('homePageNtp')]]
               </paper-radio-button>
               <paper-radio-button class="list-item" name="false">
@@ skipping 35 matching lines @@
         <template is="dom-if" if="[[showFontsPage_(currentRoute.subpage.*)]]"
             restamp>
           <settings-appearance-fonts-page prefs="{{prefs}}">
           </settings-appearance-fonts-page>
         </template>
       </neon-animatable>
     </settings-animated-pages>
   </template>
   <script src="appearance_page.js"></script>
 </dom-module>