Rework how MB and GN handle concurrent links.

build/toolchain/win/BUILD.gn

Index: build/toolchain/win/BUILD.gn
diff --git a/build/toolchain/win/BUILD.gn b/build/toolchain/win/BUILD.gn
index f7637c9fb4fdd9be04ed55c71c1910164a0aa239..097229debf87616cc22e016b2bcd112ec8e6d9ee 100644
--- a/build/toolchain/win/BUILD.gn
+++ b/build/toolchain/win/BUILD.gn
@@ -6,6 +6,7 @@ import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/win/visual_studio_version.gni")
 import("//build/toolchain/goma.gni")
 import("//build/toolchain/toolchain.gni")
+import("//build/toolchain/concurrent_links.gni")
 
 # Should only be running on Windows.
 assert(is_win)
@@ -24,9 +25,6 @@ if (use_goma) {
   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) {
@@ -50,6 +48,7 @@ if (current_toolchain == default_toolchain) {
 #  environment: File name of environment file.
 template("msvc_toolchain") {
   if (defined(invoker.concurrent_links)) {
+    # concurrent_links is picked up from the declare_arg() otherwise.
     concurrent_links = invoker.concurrent_links
   }