Material PDF: Support RTL languages

chrome/browser/resources/pdf/elements/viewer-zoom-toolbar/viewer-zoom-toolbar.css

 /* Copyright 2015 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file. */
 
 :host {
   -webkit-user-select: none;
   bottom: 0;
   padding: 48px 0;
   position: fixed;
   right: 0;
   z-index: 3;
 }
 
+:host-context([dir=rtl]) {
+  left: 0;
+  right: auto;
+}
+
 viewer-zoom-button {
   display: block;
 }
 
 /* A small gap between the zoom in/zoom out buttons. */
 #zoom-out-button {
   margin-top: 10px;
 }
 
 /* A larger gap between the fit button and bottom two buttons. */
 #zoom-in-button {
   margin-top: 24px;
 }