[MD settings] remove unwanted separator line

chrome/browser/resources/settings/settings_page/settings_subpage.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-icon/iron-icon.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-resizable-behavior/iron-resizable-behavior.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/neon-animation/neon-animatable-behavior.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-icon-button/paper-icon-button.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-ripple/paper-ripple.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-spinner/paper-spinner.html">
 <link rel="import" href="../icons.html">
 <link rel="import" href="../route.html">
 <link rel="import" href="settings_subpage_search.html">
 <link rel="import" href="../settings_shared_css.html">
 
 <dom-module id="settings-subpage">
   <template>
     <style include="settings-shared">
       :host {
         display: block;
         padding-bottom: 60px;
       }
 
+      :host-context([dir=rtl]) paper-icon-button[icon='settings:arrow-back'] {

[dschuyler, 2017/05/23 01:24:55]

This is the only place settings:arrow-back is used, so I move this mirroring
here.

+        transform: scaleX(-1);  /* Flip on the X axis (aka mirror). */
+      }
+
       #headerLine {
         min-height: 40px;
         padding-bottom: 12px;
         padding-top: 8px;
       }
 
       #learnMore {
         @apply(--cr-paper-icon-button-margin);
         align-items: center;
         display: flex;
@@ skipping 44 matching lines @@
         </settings-subpage-search>
       </template>
       <template is="dom-if" if="[[showSpinner]]">
         <paper-spinner active></paper-spinner>
       </template>
     </div>
     <content></content>
   </template>
   <script src="settings_subpage.js"></script>
 </dom-module>