Rename iterator debugging flag in GN.

tools/gn/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("//testing/test.gni")
 
 defines = [ "GN_BUILD" ]
 
 static_library("gn_lib") {
   configs += [ "//build/config:precompiled_headers" ]
@@ skipping 196 matching lines @@
     outfile,
   ]
 
   args = [
     rebase_path("//", root_build_dir),
     rebase_path(outfile, root_build_dir),
     "TOOLS_GN_LAST_COMMIT_POSITION_H_",
   ]
 }
 
+# Note for Windows debugging: GN is super-multithreaded and uses a lot of STL.
+# Iterator debugging on Windows does locking for every access, which ends up
+# slowing down debug runtime from 0:36 to 9:40. If you want to run debug builds
+# of GN over the large Chrome build, you will want to set the arg:
+#   enable_iterator_debugging = false
 executable("gn") {
   sources = [
     "gn_main.cc",
   ]
 
   deps = [
     ":gn_lib",
     ":last_commit_position",
     "//base",
   ]
@@ skipping 77 matching lines @@
 }
 
 executable("generate_test_gn_data") {
   sources = [
     "generate_test_gn_data.cc",
   ]
   deps = [
     "//base",
   ]
 }