Create a field test for sync sign in promo strings.

chrome/browser/resources/sync_setup_overlay.html

 <div id="sync-setup-overlay" class="page" hidden>
   <div id="sync-setup-login" hidden>
     <h1 id="promo-title" i18n-content="promoTitle" hidden></h1>
     <div>
       <div id="sync-setup-login-content-column">
         <h1 id="sync-setup-overlay-title"
             i18n-content="syncSetupOverlayTitle">
         </h1>
         <div class="sign-in">
           <div id="top-blurb-error" class="reset-hidden" hidden>
@@ skipping 92 matching lines @@
         <div class="action-area">
           <div class="button-strip">
             <button id="sync-setup-cancel" type="reset" i18n-content="cancel">
             </button>
           </div>
         </div>
       </div>
       <div id="sync-setup-login-promo-column" hidden>
         <h2 id="promo-message-title" i18n-content="promoMessageTitle"></h2>
         <p>
-          <span i18n-values=".innerHTML:promoMessageBody"></span>
+          <span id="sync-setup-login-promo-body"
+              i18n-values=".innerHTML:promoMessageBody"></span>
           <a id="promo-learn-more-show" i18n-content="promoLearnMoreShow"
               href="#"></a>
           <a id="promo-learn-more-hide" i18n-content="promoLearnMoreHide"
               href="#" hidden> </a>
         </p>
         <div id="promo-info-pane">
           <img id="promo-infographic"
               src="chrome://theme/IRD_SYNC_PROMO_INFO_GRAPHIC">
           <div id="promo-information" i18n-values=".innerHTML:promoInformation"
               hidden>
@@ skipping 218 matching lines @@
       <span i18n-values=".innerHTML:stopSyncingExplanation"></span>
     </div>
     <div class="action-area button-strip">
       <input id="stop-syncing-cancel" type="button"
           i18n-values="value:cancel">
       <input id="stop-syncing-ok" type="button"
           i18n-values="value:stopSyncingConfirm">
     </div>
   </div>
 </div>