[GN] Add support for generating Xcode projects.

tools/gn/BUILD.gn

Index: tools/gn/BUILD.gn
diff --git a/tools/gn/BUILD.gn b/tools/gn/BUILD.gn
index bb36b1ffe2f58f2605ce1781f02899e5a35f7017..c3b509dcdf0299110d90e2ccbdff2405fe966609 100644
--- a/tools/gn/BUILD.gn
+++ b/tools/gn/BUILD.gn
@@ -207,6 +207,10 @@ static_library("gn_lib") {
     "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",
   ]
@@ -214,6 +218,7 @@ static_library("gn_lib") {
   deps = [
     "//base",
     "//base/third_party/dynamic_annotations",
+    "//third_party/re2",
   ]
 }