Add support for VS express to the GN build.

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.
+  # 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 = ""
+
+  # 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\8.0"
 }
 
 if (visual_studio_path == "") {
   toolchain_data =
-      exec_script("../../vs_toolchain.py", [ "get_toolchain_dir" ], "value")
-  visual_studio_path = toolchain_data[0]
-  windows_sdk_path = toolchain_data[1]
+      exec_script("../../vs_toolchain.py", [ "get_toolchain_dir" ], "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
+} 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")
 }
+
+# Set when using the "Express" version of a Visual Studio version we support.
+is_visual_studio_express = (visual_studio_version == "2013e")