Use the hermetic toolchain if FORCE_MAC_TOOLCHAIN is set.

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 28 matching lines @@
   # 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
+  use_system_xcode = ""
+}
+
+if ((is_mac || is_ios) && use_system_xcode == "") {
+  _result = getenv("FORCE_MAC_TOOLCHAIN")
+  use_system_xcode = _result == ""
 }
 
 # The path to the hermetic install of Xcode. Only relevant when
 # use_system_xcode = false.
 hermetic_xcode_path =
     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.
@@ skipping 53 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}})"
 }