clang/win: use clang_warning_flags, like mac and linux do

build/set_clang_warning_flags.gypi

 # Copyright (c) 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.
 
 # This file is meant to be included to set clang-specific compiler flags.
 # To use this the following variable can be defined:
 #   clang_warning_flags:       list: Compiler flags to pass to clang.
 #   clang_warning_flags_unset: list: Compiler flags to not pass to clang.
 #
 # Only use this in third-party code. In chromium_code, fix your code to not
@@ skipping 21 matching lines @@
   'conditions': [
     ['clang==1', {
       # This uses >@ instead of @< to also see clang_warning_flags set in
       # targets directly, not just the clang_warning_flags in target_defaults.
       'cflags': [ '>@(clang_warning_flags)' ],
       'cflags!': [ '>@(clang_warning_flags_unset)' ],
       'xcode_settings': {
         'WARNING_CFLAGS': ['>@(clang_warning_flags)'],
         'WARNING_CFLAGS!': ['>@(clang_warning_flags_unset)'],
       },
+      'msvs_settings': {
+        'VCCLCompilerTool': {
+          'AdditionalOptions': [ '>@(clang_warning_flags)' ],
+          'AdditionalOptions!': [ '>@(clang_warning_flags_unset)' ],
+        },
+      },
     }],
   ],
 }