Rework how MB and GN handle concurrent links.

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/sanitizers/sanitizers.gni")
 import("//build/config/win/visual_studio_version.gni")
 import("//build/toolchain/goma.gni")
 import("//build/toolchain/toolchain.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", root_build_dir)
 
 if (use_goma) {
   goma_prefix = "$goma_dir/gomacc.exe "
 } else {
   goma_prefix = ""
 }
 
-# This value will be inherited in the toolchain below.
-concurrent_links = exec_script("../get_concurrent_links.py", [], "value")
-
 # Copy the VS runtime DLL for the default toolchain to the root build directory
 # so things will run.
 if (current_toolchain == default_toolchain) {
   if (is_debug) {
     configuration_name = "Debug"
   } else {
     configuration_name = "Release"
   }
   exec_script("../../vs_toolchain.py",
               [
@@ skipping 376 matching lines @@
 
   msvc_toolchain("winrt_x64") {
     environment = "environment.winrt_x64"
     cl = "${goma_prefix}\"${vc_bin_dir}/cl.exe\""
     is_clang = false
 
     toolchain_cpu = "x64"
     toolchain_os = current_os
   }
 }