Add a GN flag for the Windows multi-dll build.

chrome/chrome_dll.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.
 {
   'conditions': [
     ['OS=="mac" or OS=="win"', {
       'targets': [
         {
           'target_name': 'chrome_dll',
           'type': 'none',
@@ skipping 308 matching lines @@
                 },
               ],
             }],  # OS=="mac"
           ],  # conditions
         },  # target chrome_main_dll
       ],  # targets
     }],  # OS=="mac" or OS=="win"
     ['chrome_multiple_dll', {
       'targets': [
         {
+          # GN version: //chrome:chrome_child
           'target_name': 'chrome_child_dll',
           'type': 'shared_library',
           'product_name': 'chrome_child',
           'variables': {
             'enable_wexit_time_destructors': 1,
           },
           'dependencies': [
             '<@(chromium_child_dependencies)',
             '../content/content.gyp:content_app_child',
             'chrome_version_resources',
@@ skipping 36 matching lines @@
               'dependencies': [
                 '../pdf/pdf.gyp:pdf',
               ],
             }],
           ],
         },  # target chrome_child_dll
       ],
     }],
   ],
 }