Add PDF viewer example.

examples/BUILD.gn

 # Copyright 2014 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.
 
 import("//build/config/ui.gni")
 
 group("examples") {
   testonly = true
 
   deps = [
     "//examples/apptest",
     "//examples/content_handler_demo",
     "//examples/echo",
     "//examples/http_server",
+    "//examples/pdf_viewer",
     "//examples/png_viewer",
     "//examples/sample_app",
     "//examples/surfaces_app",
     "//examples/wget",
   ]
 
   if (use_aura) {
     deps += [
       "//examples/aura_demo:aura_demo",
       "//examples/browser",
       "//examples/embedded_app",
       "//examples/media_viewer",
       "//examples/nesting_app",
       "//examples/keyboard",
       "//examples/window_manager",
       "//examples/wm_flow",
     ]
   }
 }