Add to GN ppapi build.

ppapi/ppapi_host.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.
 
 {
    'targets': [
     {
+      # GN version: //ppapi:ppapi_host
       'target_name': 'ppapi_host',
       'type': '<(component)',
       'dependencies': [
         '../base/base.gyp:base',
         '../ipc/ipc.gyp:ipc',
         '../media/media.gyp:shared_memory_support',
         '../ui/surface/surface.gyp:surface',
         '../url/url.gyp:url_lib',
         'ppapi.gyp:ppapi_c',
         'ppapi_internal.gyp:ppapi_ipc',
         'ppapi_internal.gyp:ppapi_proxy',
         'ppapi_internal.gyp:ppapi_shared',
       ],
       'defines': [
         'PPAPI_HOST_IMPLEMENTATION',
       ],
       'sources': [
+        # Note: sources list duplicated in GN build.
         'host/dispatch_host_message.h',
         'host/error_conversion.cc',
         'host/error_conversion.h',
         'host/host_factory.h',
         'host/host_message_context.cc',
         'host/host_message_context.h',
         'host/instance_message_filter.cc',
         'host/instance_message_filter.h',
         'host/message_filter_host.cc',
         'host/message_filter_host.h',
         'host/ppapi_host.cc',
         'host/ppapi_host.h',
         'host/ppapi_host_export.h',
         'host/resource_host.cc',
         'host/resource_host.h',
         'host/resource_message_filter.cc',
         'host/resource_message_filter.h',
         'host/resource_message_handler.cc',
         'host/resource_message_handler.h',
       ],
     },
   ],
 }