Move use_system_xcode to //build_overrides/build.gni

.gn

 # This file is used by the GN meta build system to find the root of the source
 # tree and to set startup options. For documentation on the values set in this
 # file, run "gn help dotfile" at the command line.
 
 # The location of the build configuration file.
 buildconfig = "//build/config/BUILDCONFIG.gn"
 
 # The secondary source root is a parallel directory tree where
 # GN build files are placed when they can not be placed directly
 # in the source tree, e.g. for third party source trees.
@@ skipping 198 matching lines @@
 #      directory is updated, so adds a glob so all the files are listed as
 #      inputs. This seems to work great... until a file is deleted. When a
 #      file is deleted, all the inputs the glob lists will still be up to date
 #      and no command-lines will have been changed. The action will not be
 #      re-run and the build will be broken. It is possible to get this correct
 #      using glob, and it's possible to mess it up without glob, but globs make
 #      this situation much easier to create. if the build always lists the
 #      files and passes them to a script, it will always be correct.
 exec_script_whitelist = [
   "//android_webview/BUILD.gn",
+  "//build_overrides/build.gni",
   "//build/config/BUILD.gn",
   "//build/config/android/config.gni",
   "//build/config/android/internal_rules.gni",
   "//build/config/android/rules.gni",
   "//build/config/compiler/BUILD.gn",
   "//build/config/gcc/gcc_version.gni",
   "//build/config/ios/ios_sdk.gni",
   "//build/config/linux/BUILD.gn",
   "//build/config/linux/pkg_config.gni",
   "//build/config/linux/atk/BUILD.gn",
   "//build/config/mac/mac_sdk.gni",
   "//build/config/mac/rules.gni",
   "//build/config/posix/BUILD.gn",
   "//build/config/sysroot.gni",
   "//build/config/win/BUILD.gn",
   "//build/config/win/visual_studio_version.gni",
   "//build/toolchain/concurrent_links.gni",
   "//build/toolchain/mac/BUILD.gn",
   "//build/toolchain/nacl/BUILD.gn",
   "//build/toolchain/toolchain.gni",

[erikchen, 2016/11/11 17:00:33]

this line can be removed.

   "//build/toolchain/win/BUILD.gn",
   "//build/util/branding.gni",
   "//build/util/version.gni",
   "//chromeos/BUILD.gn",
 
   # TODO(dgn): Layer violation but breaks the build otherwise, see
   # https://crbug.com/474506
   "//clank/java/BUILD.gn",
   "//clank/native/BUILD.gn",
 
@@ skipping 10 matching lines @@
   "//third_party/skia/gn/shared_sources.gni",
 
   # CLD2 should be removed soon, delete this when we do.
   "//third_party/cld_2/BUILD.gn",
   "//tools/grit/grit_rule.gni",
 
   # Not gypi-to-gn.
   "//google_apis/BUILD.gn",
   "//printing/BUILD.gn",
 ]