MD Settings: add borders between toggle rows on basic/advanced pages

chrome/browser/resources/settings/system_page/system_page.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-flex-layout/iron-flex-layout-classes.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-button/paper-button.html">
 <link rel="import" href="/controls/controlled_button.html">
 <link rel="import" href="/controls/extension_controlled_indicator.html">
 <link rel="import" href="/controls/settings_toggle_button.html">
 <link rel="import" href="/lifetime_browser_proxy.html">
 <link rel="import" href="/prefs/prefs.html">
 <link rel="import" href="/settings_shared_css.html">
 <link rel="import" href="/system_page/system_page_browser_proxy.html">
 
 <dom-module id="settings-system-page">
   <template>
     <style include="settings-shared"></style>
-    <div class="settings-box block first">
+    <div class="settings-box first">
 <if expr="not is_macosx">
-      <settings-toggle-button
+      <settings-toggle-button class="start"
           pref="{{prefs.background_mode.enabled}}"
           label="$i18n{backgroundAppsLabel}">
       </settings-toggle-button>
+    </div>
+    <div class="settings-box">
 </if>
       <settings-toggle-button class="flex" id="hardwareAcceleration"

[dschuyler, 2017/01/13 01:48:15]

Since this is now in its own settings-box, let's make this class="start". 

[Dan Beam, 2017/01/13 03:35:44]

Done.

           pref="{{prefs.hardware_acceleration_mode.enabled}}"
           label="$i18n{hardwareAccelerationLabel}">
         <template is="dom-if" if="[[shouldShowRestart_(
             prefs.hardware_acceleration_mode.enabled.value)]]">
           <paper-button on-tap="onRestartTap_" class="more-actions">
             $i18n{restart}
           </paper-button>
         </template>
       </settings-toggle-button>
     </div>
     <div class="settings-box">
       <controlled-button class="primary-button" pref="[[prefs.proxy]]"
           on-tap="onChangeProxySettingsTap_">
         $i18n{changeProxySettings}
       </controlled-button>
     </div>
     <template is="dom-if" if="[[prefs.proxy.extensionId]]">
       <div class="settings-box continuation">
         <extension-controlled-indicator class="start"
             extension-id="[[prefs.proxy.extensionId]]"
             extension-name="[[prefs.proxy.controlledByName]]"
             extension-can-be-disabled="[[prefs.proxy.extensionCanBeDisabled]]"
             on-extension-disable="onExtensionDisable_">
         </extension-controlled-indicator>
       </div>
     </template>
   </template>
   <script src="/system_page/system_page.js"></script>
 </dom-module>