Use depot_tools toolchain, remove now unneeded code, fix not including path to win sdk tools

build/config/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")
 
-declare_args() {
-  # 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"
-}
-
 # Compiler setup for the Windows SDK. Applied to all targets.
 config("sdk") {
   # The include path is the stuff returned by the script.
   #include_dirs = msvc_config[0]  TODO(brettw) make this work.
 
   defines = [
     "_ATL_NO_OPENGL",
     "_SECURE_ATL",
     "_WIN32_WINNT=0x0602",
     "_WINDOWS",
@@ skipping 85 matching lines @@
 }
 
 # Incremental linking ----------------------------------------------------------
 
 config("incremental_linking") {
   ldflags = [ "/INCREMENTAL" ]
 }
 config("no_incremental_linking") {
   ldflags = [ "/INCREMENTAL:NO" ]
 }