Add to GN ppapi build.

ppapi/ppapi_shared.gypi

 # Copyright (c) 2012 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.
 
 {
   'target_defaults': {
     'variables': {
       'ppapi_shared_target': 0,
     },
     'target_conditions': [
       # This part is shared between the targets defined below.
       ['ppapi_shared_target==1', {
         'sources': [
+          # Note: sources list duplicated in GN build.
           'shared_impl/array_var.cc',
           'shared_impl/array_var.h',
           'shared_impl/array_writer.cc',
           'shared_impl/array_writer.h',
           'shared_impl/callback_tracker.cc',
           'shared_impl/callback_tracker.h',
           'shared_impl/compositor_layer_data.cc',
           'shared_impl/compositor_layer_data.h',
           'shared_impl/dictionary_var.cc',
           'shared_impl/dictionary_var.h',
@@ skipping 330 matching lines @@
               'thunk/ppb_tcp_socket_private_thunk.cc',
               'thunk/ppb_udp_socket_private_thunk.cc',
               'thunk/ppb_x509_certificate_private_thunk.cc',
             ],
           }],
         ],
       }],
     ],
   },
 }