Make goma work on win GN builds.

build/config/win/visual_studio_version.gni

 # Copyright 2014 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.
 
 declare_args() {
   # Path to Visual Studio. If empty, the default is used which is to use the
   # automatic toolchain in depot_tools. If set, you must also set the
   # visual_studio_version and wdk_path.
   visual_studio_path = ""
 
   # Version of Visual Studio pointed to by the visual_studio_path.
   # Use "2013" for Visual Studio 2013, or "2013e" for the Express version.
   visual_studio_version = ""
 
+  # Path to the actual binary directory for the Visual Studio toolchains.
+  vc_bin_dir = ""
+
   # Directory of the Windows driver kit. If visual_studio_path is empty, this
   # will be auto-filled.
   wdk_path = ""
 
   # Full path to the Windows SDK, not including a backslash at the end.
   # This value is the default location, override if you have a different
   # installation location.
   windows_sdk_path = "C:\Program Files (x86)\Windows Kits\8.1"
 
   # The list of include directories that are treated as "system" include
   # directories. TODO(scottmg): These are incorrectly put on the command line
   # in GN, they should really be stored into %INCLUDE%.
   system_include_dirs = []
 }
 
 if (visual_studio_path == "") {
   toolchain_data =
-      exec_script("../../vs_toolchain.py", [ "get_toolchain_dir" ], "scope")
+      exec_script("../../vs_toolchain.py",
+                  [ "get_toolchain_dir", cpu_arch ], "scope")
   visual_studio_path = toolchain_data.vs_path
   windows_sdk_path = toolchain_data.sdk_path
   visual_studio_version = toolchain_data.vs_version
   wdk_path = toolchain_data.wdk_dir
   visual_studio_runtime_dirs = toolchain_data.runtime_dirs
+  vc_bin_dir = toolchain_data.vc_bin_dir
 } else {
   assert(visual_studio_version != "",
          "You must set the visual_studio_version if you set the path")
   assert(wdk_path != "",
          "You must set the wdk_path if you set the visual studio path")
   visual_studio_runtime_dirs = []
 }
 
 # The Windows SDK include directories must be first. They both have a sal.h,
 # and the SDK one is newer and the SDK uses some newer features from it not
 # present in the Visual Studio one.
 system_include_dirs = [
   "$windows_sdk_path\Include\shared",
   "$windows_sdk_path\Include\um",
   "$windows_sdk_path\Include\winrt",
   "$visual_studio_path\VC\include",
   "$visual_studio_path\VC\atlmfc\include",
 ]