Add Now setting and migration logic.

chrome/browser/resources/options/browser_options.html

 <div id="settings" class="page" hidden>
   <header>
     <h1 i18n-content="settingsTitle"></h1>
   </header>
   <include src="reset_profile_settings_banner.html">
   <include src="automatic_settings_reset_banner.html">
 <if expr="chromeos">
   <include src="secondary_user_banner.html">
   <section>
     <div id="network-section-header" class="section-header">
@@ skipping 139 matching lines @@
         <div class="settings-row">
           <select id="default-search-engine" class="weakrtl"
               aria-labelledby="default-search-engine-label"></select>
           <span class="controlled-setting-indicator"
               pref="default_search_provider_data.template_url_data">
           </span>
           <button id="manage-default-search-engines"
               i18n-content="defaultSearchManageEngines">
           </button>
         </div>
+        <div id="google-now-launcher" hidden>
+          <div class="checkbox">
+            <label>
+              <input pref="google_now_launcher.enabled" type="checkbox">
+              <span i18n-content="googleNowLauncherEnable"></span>
+            </label>
+          </div>
+        </div>
         <div id="hotword-always-on-search" hidden>
           <div class="checkbox controlled-setting-with-label">
             <label>
               <input id="hotword-always-on-search-checkbox"
                   pref="hotword.always_on_search_enabled" type="checkbox">
               <span i18n-content="hotwordSearchEnable"></span>
             </label>
             <a target="_blank" class="hotword-link"
                 i18n-content="learnMore"
                 i18n-values="href:hotwordLearnMoreURL">
@@ skipping 845 matching lines @@
       </button>
     </div>
   </section>
   </div>  <!-- advanced-settings-container -->
   </div>  <!-- advanced-settings -->
   <footer id="advanced-settings-footer">
     <a is="action-link" id="advanced-settings-expander"
         i18n-content="showAdvancedSettings"></a>
   </footer>
 </div>