Add a variable use_system_xcode to GN.

build/toolchain/toolchain.gni

 # Copyright 2015 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.
 
 # Toolchain-related configuration that may be needed outside the context of the
 # toolchain() rules themselves.
 
 import("//build/config/chrome_build.gni")
 
 declare_args() {
@@ skipping 22 matching lines @@
   # the CLANG_REVISION to use, instead of the version hard-coded into
   # //tools/clang/scripts/update.py. This should only be used in
   # conjunction with setting LLVM_FORCE_HEAD_REVISION in the
   # environment when `gclient runhooks` is run as well.
   llvm_force_head_revision = false
 
   # Compile with Xcode version of clang instead of hermetic version shipped
   # with the build. Used on iOS to ship official builds (as they are built
   # with the version of clang shipped with Xcode).
   use_xcode_clang = is_ios && is_official_build
+
+  # Use the system install of Xcode for tools like ibtool, libtool, etc.
+  # This does not affect the compiler. When this variable is false, targets will
+  # instead use a hermetic install of Xcode. [The hermetic install can be
+  # obtained with gclient sync after setting the environment variable
+  # FORCE_MAC_TOOLCHAIN].
+  use_system_xcode = true
+
+  # The path to the hermetic install of Xcode. Only relevant when
+  # use_system_xcode = false.
+  hermetic_xcode_path =

[sdefresne, 2016/10/07 13:25:16]

nit: do you expect developer to change this variable? if not, maybe it would be
better to put it outside of "declare_args()" block

[erikchen, 2016/10/07 23:25:41]

I expect to use this when I rev the hermetic toolchain. Not sure about others...

+      rebase_path("//build/mac_files/Xcode.app", "", root_build_dir)
 }
 
 declare_args() {
   if (is_clang) {
     # Clang compiler version. Clang files are placed at version-dependent paths.
     clang_version = "4.0.0"
   }
 }
 
 # Check target_os here instead of is_ios as this file is loaded for secondary
@@ skipping 48 matching lines @@
   _tool_wrapper_path =
       rebase_path("//build/toolchain/win/tool_wrapper.py", root_build_dir)
 
   stamp_command = "$python_path $_tool_wrapper_path stamp {{output}}"
   copy_command =
       "$python_path $_tool_wrapper_path recursive-mirror {{source}} {{output}}"
 } else {
   stamp_command = "touch {{output}}"
   copy_command = "ln -f {{source}} {{output}} 2>/dev/null || (rm -rf {{output}} && cp -af {{source}} {{output}})"
 }