Add Roboto-Medium webfont and factor out common Roboto WebUI CSS file.

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. */
 
-@font-face {
-  font-family: 'Roboto';
-  font-style: normal;
-  font-weight: 400;
-  src: local('Roboto Regular'), local('Roboto-Regular'), url(chrome://resources/roboto/roboto.woff2) format('woff2');
-  unicode-range: U+0460-052F, U+20B4, U+2DE0-2DFF, U+A640-A69F;
-}
-
 body {
   background-color: #424242;
   font-family: Roboto, 'Helvetica Neue', Helvetica, Arial;
   margin: 0;
 }
 
 viewer-pdf-toolbar {
   position: fixed;
   width: 100%;
   /* TODO(alexandrec): the viewer-pane's paper-dialog is set to z-index: 12
@@ skipping 30 matching lines @@
 
 viewer-error-screen {
   visibility: hidden;
   z-index: 2;
 }
 
 viewer-password-screen {
   visibility: hidden;
   z-index: 2;
 }