Add more GN deps to the build.

device/media_transfer_protocol/media_transfer_protocol.gyp

 # Copyright 2013 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.
 
 {
   'variables': {
     'chromium_code': 1,
   },
   'targets': [
     {
       # Protobuf compiler / generator for the MtpFileEntry and
       # MtpFileEntries protocol buffers.
+      # GN version: //device/media_transfer_protocol:mtp_file_entry_proto
       'target_name': 'mtp_file_entry_proto',
       'type': 'static_library',
       'sources': [
         '../../third_party/cros_system_api/dbus/mtp_file_entry.proto',
       ],
       'variables': {
         'proto_in_dir': '../../third_party/cros_system_api/dbus',
         'proto_out_dir': 'device/media_transfer_protocol',
       },
       'includes': ['../../build/protoc.gypi'],
     },
     {
       # Protobuf compiler / generator for the MtpStorageInfo protocol
       # buffer.
+      # GN version: //device/media_transfer_protocol:mtp_storage_info_proto
       'target_name': 'mtp_storage_info_proto',
       'type': 'static_library',
       'sources': [
         '../../third_party/cros_system_api/dbus/mtp_storage_info.proto',
       ],
       'variables': {
         'proto_in_dir': '../../third_party/cros_system_api/dbus',
         'proto_out_dir': 'device/media_transfer_protocol',
       },
       'includes': ['../../build/protoc.gypi'],
     },
     {
+      # GN version: //device/media_transfer_protocol
       'target_name': 'device_media_transfer_protocol',
       'type': 'static_library',
       'dependencies': [
         '../../build/linux/system.gyp:dbus',
         'mtp_file_entry_proto',
         'mtp_storage_info_proto',
       ],
       'sources': [
         'media_transfer_protocol_daemon_client.cc',
         'media_transfer_protocol_daemon_client.h',
         'media_transfer_protocol_manager.cc',
         'media_transfer_protocol_manager.h',
       ],
     },
   ],
 }