[GN] Add support for generating Xcode projects.

tools/gn/gn.gyp

 {
   'variables': {
     'chromium_code': 1,
   },
   'targets': [
     {
       'target_name': 'gn_lib',
       'type': 'static_library',
       'dependencies': [
         '../../base/base.gyp:base',
@@ skipping 189 matching lines @@
         'value_extractors.cc',
         'value_extractors.h',
         'variables.cc',
         'variables.h',
         'visibility.cc',
         'visibility.h',
         'visual_studio_utils.cc',
         'visual_studio_utils.h',
         'visual_studio_writer.cc',
         'visual_studio_writer.h',
+        'xcode_object.cc',
+        'xcode_object.h',
+        'xcode_writer.cc',
+        'xcode_writer.h',
         'xml_element_writer.cc',
         'xml_element_writer.h',
       ],
     },
     {
       'target_name': 'gn',
       'type': 'executable',
       'sources': [
         'gn_main.cc',
       ],
@@ skipping 81 matching lines @@
           'dependencies': [
             'gn_unittests',
           ],
           'includes': [ '../../build/isolate.gypi' ],
           'sources': [ 'gn_unittests.isolate' ],
         },
       ],
     }],
   ],
 }