Create a field test for sync sign in promo strings.

chrome/browser/resources/sync_setup_overlay.js

Index: chrome/browser/resources/sync_setup_overlay.js
===================================================================
--- chrome/browser/resources/sync_setup_overlay.js	(revision 111699)
+++ chrome/browser/resources/sync_setup_overlay.js	(working copy)
@@ -732,6 +732,16 @@
       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');
@@ -882,6 +892,10 @@
     SyncSetupOverlay.getInstance().showStopSyncingUI_();
   };
 
+  SyncSetupOverlay.populatePromoMessage = function(resName) {
+    SyncSetupOverlay.getInstance().populatePromoMessage_(resName);
+  };
+
   // Export
   return {
     SyncSetupOverlay: SyncSetupOverlay