In Sync UI config, reset the "custom passphrase" div to hidden.

chrome/browser/resources/options/sync_setup_overlay.html

 <div id="sync-setup-overlay" class="page" hidden>
   <div id="sync-setup-login" hidden>
     <h1 i18n-content="syncSetupOverlayTitle"></h1>
     <div class="content-area">
       <div>
         <!-- top-blurb-error's reset state is handled differently from most
              elements. Instead of being [hidden] it is visibility:hidden so it
              still takes up space. But it is also [hidden] when we display the
              captcha, so when it is reset, it needs to be shown. -->
         <div id="top-blurb-error"
@@ skipping 334 matching lines @@
               <label for="google-option" i18n-content="googleOption"></label>
             </div>
             <div>
               <input id="explicit-option" name="option" type="radio"
                   value="explicit">
               <label for="explicit-option" i18n-content="explicitOption">
               </label>
               <a i18n-values="href:encryptionHelpURL" target="_blank"
                   i18n-content="learnMore"></a>
             </div>
-            <div id="sync-custom-passphrase" hidden>
+            <div id="sync-custom-passphrase" 
+                class="reset-hidden" hidden>
               <div id="sync-passphrase-message">
                 <span i18n-content="sectionExplicitMessagePrefix"></span>
                 <a href="http://google.com/dashboard" target="_blank"
                     i18n-content="sectionExplicitMessagePostfix"></a>
                 <span>.</span>
               </div>
               <div class="sync-custom-passphrase-input">
                 <input id="custom-passphrase" type="password"
                     class="reset-value"
                     i18n-values="placeholder:passphraseLabel">
@@ skipping 50 matching lines @@
       <span i18n-content="stopSyncingExplanation"></span>
     </div>
     <div class="action-area">
       <input id="stop-syncing-ok" type="button"
         i18n-values="value:stopSyncingConfirm">
       <input id="stop-syncing-cancel" type="button"
         i18n-values="value:cancel">
     </div>
   </div>
 </div>