Fix GN mac toolchain error.

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")
 
@@ skipping 177 matching lines @@
       description = "STAMP {{output}}"
     }
 
     tool("copy") {
       command = "ln -f {{source}} {{output}} 2>/dev/null || (rm -rf {{output}} && cp -af {{source}} {{output}})"
       description = "COPY {{source}} {{output}}"
     }
 
     toolchain_args() {
       current_os = invoker.toolchain_os
-
-      # TODO(dpranke): os is here for backwards compatibility.
-      os = current_os
     }
   }
 }
 
 # Toolchain representing the target build (either mac or iOS).
 mac_clang_toolchain("clang") {
   toolchain_os = current_os
 }
 
 # This toolchain provides a way for iOS target compiles to reference targets
 # compiled for the host system. It just overrides the OS back to "mac".
 mac_clang_toolchain("host_clang") {
   toolchain_os = "mac"
 }