[MD settings] replace secondary-action with separator

chrome/browser/resources/settings/search_page/search_page.html

 <link rel="import" href="chrome://resources/html/i18n_behavior.html">
 <link rel="import" href="chrome://resources/html/md_select_css.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/neon-animation/neon-animatable.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-button/paper-button.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-icon-button/paper-icon-button-light.html">
 <link rel="import" href="../controls/extension_controlled_indicator.html">
 <link rel="import" href="../controls/settings_toggle_button.html">
 <link rel="import" href="../icons.html">
 <link rel="import" href="../i18n_setup.html">
@@ skipping 70 matching lines @@
         <template is="dom-if" if="[[hotwordInfo_.allowed]]">
           <!-- Hotword (OK Google) -->
           <div class="settings-box two-line continuation indented">
             <settings-toggle-button id="hotwordSearchEnable" class="start"
                 pref="{{hotwordSearchEnablePref_}}"
                 label="$i18n{searchOkGoogleLabel}"
                 sub-label="[[getHotwordSearchEnableSubLabel_(
                            hotwordInfo_.alwaysOn)]]"
                 on-change="onHotwordSearchEnableChange_">
             </settings-toggle-button>
-            <div class="secondary-action"
+            <div class="layout horizontal center"

[hcarmona, 2017/05/01 18:12:30]

Why don't other sections need these changes?

[dschuyler, 2017/05/01 19:12:08]

This is here for the sake of making a group to hide/show with the hidden$=
attribute. I would like to remove the dependency here and elsewhere on the
layout... classes. I'll look at that as another CL.

                 hidden$="[[!getShowHotwordSearchRetrain_(hotwordInfo_.*)]]">
+              <div class="separator"></div>
               <paper-button on-tap="onRetrainTap_" class="secondary-button">
                 $i18n{searchOkGoogleRetrain}
               </paper-button>
             </div>
           </div>
 
           <template is="dom-if"
               if="[[getShowHotwordError_(hotwordInfo_.*,
                   hotwordSearchEnablePref_)]]">
             <div class="settings-box continuation indented">
@@ skipping 34 matching lines @@
         <settings-subpage
             associated-control="[[$$('#subpage-trigger')]]"
             page-title="$i18n{searchEnginesManage}">
           <settings-search-engines-page></settings-search-engines-page>
         </settings-subpage>
       </template>
     </settings-animated-pages>
   </template>
   <script src="search_page.js"></script>
 </dom-module>