Move the common child process messages into their own file.

content/content_common.gypi

 # Copyright (c) 2011 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': [
     {
       'target_name': 'content_common',
       'type': '<(library)',
       'dependencies': [
         '../ipc/ipc.gyp:ipc',
       ],
       'include_dirs': [
         '..',
       ],
       'sources': [
+        'common/child_process_messages.h',
         'common/common_param_traits.cc',
         'common/common_param_traits.h',
         'common/content_message_generator.cc',
         'common/content_message_generator.h',
         'common/content_constants.cc',
         'common/content_constants.h',
         'common/content_switches.cc',
         'common/content_switches.h',
         'common/file_system_messages.h',
         'common/message_router.cc',
@@ skipping 20 matching lines @@
         'common/socket_stream_messages.h',
       ],
       'conditions': [
         ['OS=="win"', {
           'msvs_guid': '062E9260-304A-4657-A74C-0D3AA1A0A0A4',
         }],
       ],
     },
   ],
 }