[MD settings] mirror subpage back arrow in rtl

chrome/browser/resources/settings/settings_shared_css.html

 <link rel="import" href="chrome://resources/cr_elements/shared_style_css.html">
 <link rel="import" href="/settings_vars_css.html">
 
 <!-- Common styles for Material Design settings. -->
 <dom-module id="settings-shared">
   <template>
     <style include="cr-shared-style">
       button[is='paper-icon-button-light'] {
         --paper-icon-button-light-ripple: {
           /* Center the ripple on the icon button. */
@@ skipping 10 matching lines @@
       }
 
       [actionable] button[is="paper-icon-button-light"].icon-arrow-right {
         background-image: url(images/arrow_right.svg);
       }
 
       [actionable] button[is="paper-icon-button-light"].icon-external {
         background-image: url(images/open_in_new.svg);
       }
 
+      :host-context([dir=rtl]) paper-icon-button[icon='settings:arrow-back'] {
+        transform: scaleX(-1);  /* Flip on the X axis (aka mirror). */
+      }
+
       /* Use <h2> as the "sub-header" mentioned in the UX design docs. */
       h2 {
         align-items: center;
         align-self: flex-start;
         color: var(--paper-grey-600);
         display: flex;
         font-size: inherit;
         font-weight: 500;
         margin: 0;
         padding-top: 24px;
@@ skipping 383 matching lines @@
       /* Turns the arrow direction downwards, when the bubble is placed above
        * the anchor element */
       .search-bubble-innards.above::after {
         -webkit-transform: rotate(-135deg);
         bottom: -5px;
         top: auto;
       }
     </style>
   </template>
 </dom-module>