Get `gn gen` to succeed on Windows

build/toolchain/win/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.
 
 import("//build/config/win/visual_studio_version.gni")
 import("//build/toolchain/goma.gni")
 
 # Should only be running on Windows.
 assert(is_win)
 
 # Setup the Visual Studio state.
 #
 # Its arguments are the VS path and the compiler wrapper tool. It will write
 # "environment.x86" and "environment.x64" to the build directory and return a
 # list to us.
-gyp_win_tool_path = rebase_path("//tools/gyp/pylib/gyp/win_tool.py",
+gyp_win_tool_path = rebase_path("//build/win/win_tool.py",
                                 root_build_dir)
 exec_script("setup_toolchain.py",
             [ visual_studio_path, gyp_win_tool_path, windows_sdk_path ])
 
 # This value will be inherited in the toolchain below.
 concurrent_links = exec_script("../get_concurrent_links.py", [], "value")
 
 # Parameters:
 #  cpu_arch: cpu_arch to pass as a build arg
 #  environment: File name of environment file.
@@ skipping 140 matching lines @@
 msvc_toolchain("32") {
   environment = "environment.x86"
   cpu_arch = "x64"
 }
 
 msvc_toolchain("64") {
   environment = "environment.x64"
   cpu_arch = "x64"
   force_win64 = true
 }