Make goma work on win GN builds.

build/config/win/visual_studio_version.gni

Index: build/config/win/visual_studio_version.gni
diff --git a/build/config/win/visual_studio_version.gni b/build/config/win/visual_studio_version.gni
index 24c00cd6d98a68e9e5a7faf3f8cd16b39c96fecb..27cf31aec896820a5629bcd940d019a9ae009f09 100644
--- a/build/config/win/visual_studio_version.gni
+++ b/build/config/win/visual_studio_version.gni
@@ -12,6 +12,9 @@ declare_args() {
   # 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 = ""
@@ -29,12 +32,14 @@ declare_args() {
 
 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")