Material PDF: Variety of minor UI bugfixes

chrome/browser/resources/pdf/index-material.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. */
 
 body {
   background-color: rgb(82, 86, 89);
   font-family: 'Roboto', 'Noto', sans-serif;
   margin: 0;
 }
 
 viewer-pdf-toolbar {
   position: fixed;
   width: 100%;
-  z-index: 3;
+  z-index: 4;
 }
 
 #plugin {
   height: 100%;
   position: fixed;
   width: 100%;
   z-index: 1;
 }
 
 #sizer {
   position: absolute;
   z-index: 0;
 }