Test CL to demonstrate GN + hermetic toolchain

build/toolchain/toolchain.gni

Index: build/toolchain/toolchain.gni
diff --git a/build/toolchain/toolchain.gni b/build/toolchain/toolchain.gni
index 6251fe05947f988ceff0163112de1da0f7c751a6..292723654b6b6eaa1497812688903a47a799a413 100644
--- a/build/toolchain/toolchain.gni
+++ b/build/toolchain/toolchain.gni
@@ -40,6 +40,19 @@ declare_args() {
   # 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 =
+      rebase_path("//build/mac_files/Xcode.app", "", root_build_dir)
+  hermetic_ibtool_path = rebase_path("Contents/Developer/usr/bin/ibtool", "", hermetic_xcode_path)
 }
 
 declare_args() {