Allow open local pdf file from FileBrowse UI.

chrome/browser/resources/shared/js/media_common.js

 // Copyright (c) 2010 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.
 
 function pathIsVideoFile(path) {
   return /\.(mp4|ogg|mpg|avi|mov)$/i.test(path);
 }
 
 function pathIsAudioFile(path) {
   return /\.(mp3|m4a|wav)$/i.test(path);
 }
 
 function pathIsImageFile(path) {
   return /\.(jpg|png|gif)$/i.test(path);
 }
 
 function pathIsHtmlFile(path) {
   return /\.(htm|html|txt)$/i.test(path);
 }
+
+function pathIsPdfFile(path) {
+  return /\.(pdf)$/i.test(path);
+}