Enables GN build on windows

build/config/BUILDCONFIG.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.
 
 # =============================================================================
 # PLATFORM SELECTION
 # =============================================================================
 #
 # There are two main things to set: "os" and "cpu". The "toolchain" is the name
 # of the GN thing that encodes combinations of these things.
@@ skipping 319 matching lines @@
 # TOOLCHAIN SETUP
 # ==============================================================================
 #
 # Here we set the default toolchain, as well as the variable host_toolchain
 # which will identify the toolchain corresponding to the local system when
 # doing cross-compiles. When not cross-compiling, this will be the same as the
 # default toolchain.
 
 if (is_win) {
   # On windows we use the same toolchain for host and target by default.
-  if (is_clang) {
-    host_toolchain = "//build/toolchain/win:clang_$current_cpu"
-  } else {
-    host_toolchain = "//build/toolchain/win:$current_cpu"
-  }
+  host_toolchain = "//build/toolchain/win:$current_cpu"
   set_default_toolchain("$host_toolchain")
 } else if (is_android) {
   if (host_os == "linux") {
     if (host_cpu == "x86" || host_cpu == "x64") {
       host_toolchain = "//build/toolchain/linux:clang_$host_cpu"
     } else {
       host_toolchain = "//build/toolchain/linux:$host_cpu"
     }
   } else if (host_os == "mac") {
     host_toolchain = "//build/toolchain/mac:clang_$host_cpu"
@@ skipping 111 matching lines @@
       sources = invoker.sources
     }
     if (defined(invoker.testonly)) {
       testonly = invoker.testonly
     }
     if (defined(invoker.visibility)) {
       visibility = invoker.visibility
     }
   }
 }