Remove GN forwarding of the target variables.

build/toolchain/mac/BUILD.gn

 # Copyright (c) 2013 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.
 
 # TODO(brettw) Use "gcc_toolchain.gni" like the Linux toolchains. This requires
 # some enhancements since the commands on Mac are slightly different than on
 # Linux.
 
 import("../goma.gni")
 import("//build/config/clang/clang.gni")
@@ skipping 55 matching lines @@
 template("mac_toolchain") {
   toolchain(target_name) {
     # When invoking this toolchain not as the default one, these args will be
     # passed to the build. They are ignored when this is the default toolchain.
     assert(defined(invoker.toolchain_args),
            "Toolchains must declare toolchain_args")
     toolchain_args = {
       # Populate toolchain args from the invoker.
       forward_variables_from(invoker.toolchain_args, "*")
 
-      # These values need to be passed through unchanged to all secondary
-      # toolchains. BUILDCONFIG.gn sets some defaults based on the values of
-      # the operating system and compiler, and we want to force the values to
-      # be consistent if re-running the computation in another context leads
-      # to different defaults.
+      # The host toolchain value computed by the default toolchain's setup
+      # needs to be passed through unchanged to all secondary toolchains to
+      # ensure that it's always the same, regardless of the values that may be
+      # set on those toolchains.
       host_toolchain = host_toolchain
-      target_os = target_os
-      target_cpu = target_cpu
     }
 
     # When the invoker has explicitly overridden use_goma or cc_wrapper in the
     # toolchain args, use those values, otherwise default to the global one.
     # This works because the only reasonable override that toolchains might
     # supply for these values are to force-disable them.
     if (defined(toolchain_args.use_goma)) {
       toolchain_uses_goma = toolchain_args.use_goma
     } else {
       toolchain_uses_goma = use_goma
@@ skipping 359 matching lines @@
     }
   }
 
   mac_toolchain("ios_clang_x64") {
     toolchain_args = {
       current_cpu = "x64"
       current_os = "ios"
     }
   }
 }