Update from https://crrev.com/317530

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 13 matching lines @@
 # list to us.
 gyp_win_tool_path =
     rebase_path("//tools/gyp/pylib/gyp/win_tool.py", root_build_dir)
 
 toolchain_data = exec_script("setup_toolchain.py",
                              [
                                visual_studio_path,
                                gyp_win_tool_path,
                                windows_sdk_path,
                                visual_studio_runtime_dirs,
-                               cpu_arch,
+                               current_cpu,
                              ],
                              "scope")
 
 if (vc_bin_dir == "") {
   vc_bin_dir = toolchain_data.vc_bin_dir
 }
 
 # 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
+#  current_cpu: current_cpu 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.cpu_arch,
+                invoker.current_cpu,
               ])
 
   if (use_goma) {
     goma_prefix = "$goma_dir/gomacc.exe "
   } else {
     goma_prefix = ""
   }
 
   cl = "${goma_prefix}\"${vc_bin_dir}/cl.exe\""
 
@@ skipping 117 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
+      current_cpu = invoker.current_cpu
+
+      # TODO(dpranke): cpu_arch is here for backwards compatibility.
+      cpu_arch = current_cpu
     }
   }
 }
 
 # 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") {
+if (current_cpu == "x86") {
   msvc_toolchain("32") {
     environment = "environment.x86"
-    cpu_arch = "x86"
+
+    current_cpu = "x86"
   }
 }
 
-if (cpu_arch == "x64") {
+if (current_cpu == "x64") {
   msvc_toolchain("64") {
     environment = "environment.x64"
-    cpu_arch = "x64"
+
+    current_cpu = "x64"
   }
 }