[GN] Add support for generating Xcode projects.

tools/gn/BUILD.gn

 # Copyright (c) 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.
 
 import("//testing/test.gni")
 
 defines = [ "GN_BUILD" ]
 
 static_library("gn_lib") {
   configs += [ "//build/config:precompiled_headers" ]
@@ 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",
   ]
 
   deps = [
     "//base",
     "//base/third_party/dynamic_annotations",
   ]
 }
 
@@ skipping 102 matching lines @@
     "format_test_data/",
   ]
 
   deps = [
     ":gn_lib",
     "//base/test:run_all_unittests",
     "//base/test:test_support",
     "//testing/gtest",
   ]
 }