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

BUG=589630

Review URL: https://codereview.chromium.org/1798133002

Cr-Commit-Position: refs/heads/master@{#381754}

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

@@ -49,6 +49,7 @@
         </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>

chrome/browser/resources/settings/appearance_page/appearance_page.js

@@ -129,6 +129,18 @@ Polymer({
                         this.setResetThemeEnabled.bind(this));
   },
 
+  /**
+   * @param {boolean} isNtp Whether to use the NTP as the home page.
+   * @param {string} homepage If not using NTP, use this URL.
+   * @return {string} The sub-label.
+   * @private
+   */
+  getShowHomeSubLabel_: function(isNtp, homepage) {
+    if (isNtp)
+      return this.i18n('homePageNtp');
+    return homepage || this.i18n('exampleDotCom');
+  },
+
   /**
    * @param {boolean} enabled Whether the theme reset is available.
    */