Update from https://crrev.com/304418

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/win/visual_studio_version.gni")
 import("//build/toolchain/goma.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)
 exec_script("setup_toolchain.py",
             [ visual_studio_path, gyp_win_tool_path, windows_sdk_path ])
 
 # This value will be inherited in the toolchain below.
 concurrent_links = exec_script("../get_concurrent_links.py", [], "value")
 
 # Parameters:
 #  cpu_arch: cpu_arch to pass as a build arg
 #  environment: File name of environment file.
-#  force_win64 (optional): value for this build arg.
 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.cpu_arch])
+
   toolchain(target_name) {
     # Make these apply to all tools below.
     lib_switch = ""
     lib_dir_switch="/LIBPATH:"
 
     tool("cc") {
       rspfile = "{{output}}.rsp"
       pdbname = "{{target_out_dir}}/{{target_output_name}}_c.pdb"
       command = "ninja -t msvc -e $env -- cl.exe /nologo /showIncludes /FC @$rspfile /c {{source}} /Fo{{output}} /Fd$pdbname"
       depsformat = "msvc"
@@ skipping 102 matching lines @@
 
     tool("copy") {
       command =  "$python_path gyp-win-tool recursive-mirror {{source}} {{output}}"
       description = "COPY {{source}} {{output}}"
     }
 
     # When invoking this toolchain not as the default one, these args will be
     # passed to the build. They are ignored when this is the default toolchain.
     toolchain_args() {
       cpu_arch = invoker.cpu_arch
-
-      # Normally the build config resets the CPU architecture to 32-bits. Setting
-      # this flag overrides that behavior.
-      if (defined(invoker.force_win64)) {
-        force_win64 = invoker.force_win64
-      }
     }
   }
 }
 
-msvc_toolchain("32") {
-  environment = "environment.x86"
-  cpu_arch = "x64"
+# TODO(dpranke): Declare both toolchains all of the time when we
+# get it sorted out how we want to support them both in a single build.
+# Right now only one of these can be enabled at a time because the
+# runtime libraries get copied to root_build_dir and would collide.
+if (cpu_arch == "x86") {
+  msvc_toolchain("32") {
+    environment = "environment.x86"
+    cpu_arch = "x86"
+  }
 }
 
-msvc_toolchain("64") {
-  environment = "environment.x64"
-  cpu_arch = "x64"
-  force_win64 = true
+if (cpu_arch == "x64") {
+  msvc_toolchain("64") {
+    environment = "environment.x64"
+    cpu_arch = "x64"
+  }
 }