Update Chomium's build files to work w/ latest GN binaries.

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 186 matching lines @@
       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 = os
+  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"
 }