Update Chomium's build files to work w/ latest GN binaries.

build/toolchain/win/midl.gni

 # Copyright 2014 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.
 
 assert(is_win)
 
 import("//build/config/win/visual_studio_version.gni")
 
 # This template defines a rule to invoke the MS IDL compiler.
 #
@@ skipping 38 matching lines @@
 
     # Note that .tlb is not included in the outputs as it is not always
     # generated depending on the content of the input idl file.
     outputs = [
       "$out_dir/$header_file",
       "$out_dir/$dlldata_file",
       "$out_dir/$interface_identifier_file",
       "$out_dir/$proxy_file",
     ]
 
-    if (cpu_arch == "x86") {
+    if (current_cpu == "x86") {
       win_tool_arch = "environment.x86"
       idl_target_platform = "win32"
-    } else if (cpu_arch == "x64") {
+    } else if (current_cpu == "x64") {
       win_tool_arch = "environment.x64"
       idl_target_platform = "x64"
     } else {
       assert(false, "Need environment for this arch")
     }
 
     args = [
       "midl-wrapper",
       win_tool_arch,
       rebase_path(out_dir, root_build_dir),
@@ skipping 18 matching lines @@
 
     # We only compile the IID files from the IDL tool rather than all outputs.
     sources = process_file_template(invoker.sources,
                                     [ "$out_dir/$interface_identifier_file" ])
 
     deps = [
       ":$action_name",
     ]
   }
 }