Make GN write the platform for MSVS GYP files.

tools/gn/secondary/base/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.
 
 component("base") {
   external = true
   sources = [
     "../build/build_config.h",
     "third_party/dmg_fp/dmg_fp.h",
     "third_party/dmg_fp/g_fmt.cc",
@@ skipping 541 matching lines @@
     "threading/thread.h",
     "threading/thread_checker.h",
     "threading/thread_checker_impl.cc",
     "threading/thread_checker_impl.h",
     "threading/thread_collision_warner.cc",
     "threading/thread_collision_warner.h",
     "threading/thread_id_name_manager.cc",
     "threading/thread_id_name_manager.h",
     "threading/thread_local.h",
     "threading/thread_local_posix.cc",
+    "threading/thread_local_storage.cc",
     "threading/thread_local_storage.h",
     "threading/thread_local_storage_posix.cc",
     "threading/thread_local_storage_win.cc",
     "threading/thread_local_win.cc",
     "threading/thread_restrictions.h",
     "threading/thread_restrictions.cc",
     "threading/watchdog.cc",
     "threading/watchdog.h",
     "threading/worker_pool.h",
     "threading/worker_pool.cc",
@@ skipping 454 matching lines @@
 static_library("run_all_unittests") {
   external = true
   sources = [
     "test/run_all_unittests.cc",
   ]
   deps = [
     ":test_support_base",
   ]
 }