Clang style plugin: add warn-only option and use it on Windows

tools/clang/plugins/Options.h

 // Copyright (c) 2012 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.
 
 #ifndef TOOLS_CLANG_PLUGINS_OPTIONS_H_
 #define TOOLS_CLANG_PLUGINS_OPTIONS_H_
 
 namespace chrome_checker {
 
 struct Options {
   Options()
       : check_base_classes(false),
         check_enum_last_value(false),
         with_ast_visitor(false),
-        check_templates(false) {}
+        check_templates(false),
+        warn_only(false) {}
 
   bool check_base_classes;
   bool check_enum_last_value;
   bool with_ast_visitor;
   bool check_templates;
+  bool warn_only;
 };
 
 }  // namespace chrome_checker
 
 #endif  // TOOLS_CLANG_PLUGINS_OPTIONS_H_