Copy only the DLLs for the default toolchain 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.
 
 declare_args() {
   # Path to the directory containing the VC binaries for the right
   # combination of host and target architectures. Currently only the
   # 64-bit host toolchain is supported, with either 32-bit or 64-bit targets.
   # If vc_bin_dir is not specified on the command line (and it normally
   # isn't), we will dynamically determine the right value to use at runtime.
@@ skipping 30 matching lines @@
 
 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 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",
+              [
+                "copy_dlls",
+                rebase_path(root_build_dir),
+                configuration_name,
+                target_cpu,
+              ])
+}
+
 # Parameters:
 #  current_cpu: current_cpu to pass as a build arg
 #  current_os: current_os to pass as a build arg
 #  environment: File name of environment file.
 template("msvc_toolchain") {
   if (defined(invoker.concurrent_links)) {
     concurrent_links = invoker.concurrent_links
   }
 
   env = invoker.environment
 
-  if (is_debug) {
-    configuration = "Debug"
-  } else {
-    configuration = "Release"
-  }
-  exec_script("../../vs_toolchain.py",
-              [
-                "copy_dlls",
-                rebase_path(root_build_dir),
-                configuration,
-                invoker.current_cpu,
-              ])
-
   cl = invoker.cl
 
   toolchain(target_name) {
     # Make these apply to all tools below.
     lib_switch = ""
     lib_dir_switch = "/LIBPATH:"
 
     tool("cc") {
       rspfile = "{{output}}.rsp"
       precompiled_header_type = "msvc"
@@ skipping 183 matching lines @@
 }
 
 msvc_toolchain("winrt_x64") {
   environment = "environment.winrt_x64"
   cl = "${goma_prefix}\"${vc_bin_dir}/cl.exe\""
   is_clang = false
 
   current_cpu = "x64"
   current_os = current_os
 }