Create a field test for sync sign in promo strings.

chrome/browser/resources/sync_setup_overlay.js

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 cr.define('options', function() {
   const OptionsPage = options.OptionsPage;
 
   // Variable to track if a captcha challenge was issued. If this gets set to
   // true, it stays that way until we are told about successful login from
   // the browser.  This means subsequent errors (like invalid password) are
@@ skipping 714 matching lines @@
     },
 
     matchesASPRegex_: function(toMatch) {
       var noSpaces = /[a-z]{16}/;
       var withSpaces = /([a-z]{4}\s){3}[a-z]{4}/;
       if (toMatch.match(noSpaces) || toMatch.match(withSpaces))
         return true;
       return false;
     },
 
+    populatePromoMessage_: function(resName) {
+      // Fetch the strings associated with |resName| and set it if it's not
+      // empty. If it's empty, we'll stick with the default text.
+      var text = localStrings.getString(resName);
+      if (!text || !text.length) {
+        return;
+      }
+      $('sync-setup-login-promo-body').textContent = text;
+    },
+
     setErrorVisibility_: function() {
       this.resetErrorVisibility_();
       var f = $('gaia-login-form');
       var email = $('gaia-email');
       var passwd = $('gaia-passwd');
       if (null == email.value || "" == email.value) {
         $('errormsg-0-email').hidden = false;
         this.setBlurbError_();
         return false;
       }
@@ skipping 130 matching lines @@
   };
 
   SyncSetupOverlay.showSuccessAndSettingUp = function() {
     SyncSetupOverlay.getInstance().showSuccessAndSettingUp_();
   };
 
   SyncSetupOverlay.showStopSyncingUI = function() {
     SyncSetupOverlay.getInstance().showStopSyncingUI_();
   };
 
+  SyncSetupOverlay.populatePromoMessage = function(resName) {
+    SyncSetupOverlay.getInstance().populatePromoMessage_(resName);
+  };
+
   // Export
   return {
     SyncSetupOverlay: SyncSetupOverlay
   };
 });