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

build/common.gypi

 # 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.
 
 # IMPORTANT:
 # Please don't directly include this file if you are building via gyp_chromium,
 # since gyp_chromium is automatically forcing its inclusion.
 {
   # Variables expected to be overriden on the GYP command line (-D) or by
   # ~/.gyp/include.gypi.
@@ skipping 2150 matching lines @@
               'variables': {
                 'conditions': [
                   ['OS=="mac" or OS=="ios"', {
                     'clang_lib_path%': '<!(cd <(DEPTH) && pwd -P)/third_party/llvm-build/Release+Asserts/lib/libFindBadConstructs.dylib',
                   }, { # OS != "mac" or OS != "ios"
                     'clang_lib_path%': '<!(cd <(DEPTH) && pwd -P)/third_party/llvm-build/Release+Asserts/lib/libFindBadConstructs.so',
                   }],
                 ],
               },
               'clang_dynlib_flags%': '-Xclang -load -Xclang <(clang_lib_path) ',
+              'clang_plugin_args%': '',
             }, { # OS == "win"
               # On Windows, the plugin is built directly into clang, so there's
               # no need to load it dynamically.
               'clang_dynlib_flags%': '',
+
+              # Don't error on plugin warnings on Windows until pre-existing warnings
+              # are cleaned up.  https://crbug.com/467287
+              'clang_plugin_args%': '-Xclang -plugin-arg-find-bad-constructs -Xclang warn-only',
             }]
           ],
         },
         # If you change these, also change build/config/clang/BUILD.gn.
         'clang_chrome_plugins_flags%':
           '<(clang_dynlib_flags)'
-          '-Xclang -add-plugin -Xclang find-bad-constructs',
+          '-Xclang -add-plugin -Xclang find-bad-constructs '
+          '<(clang_plugin_args)',
       }],
       ['asan==1 or msan==1 or lsan==1 or tsan==1', {
         'clang%': 1,
         'use_allocator%': 'none',
         'use_sanitizer_options%': 1,
         # Disable ICF in the linker to avoid debug info loss.
         'gold_icf_level%': 'none',
       }],
       ['asan==1 and OS=="linux" and chromeos==0', {
         'use_custom_libcxx%': 1,
@@ skipping 3964 matching lines @@
     # settings in target dicts.  SYMROOT is a special case, because many other
     # Xcode variables depend on it, including variables such as
     # PROJECT_DERIVED_FILE_DIR.  When a source group corresponding to something
     # like PROJECT_DERIVED_FILE_DIR is added to a project, in order for the
     # files to appear (when present) in the UI as actual files and not red
     # red "missing file" proxies, the correct path to PROJECT_DERIVED_FILE_DIR,
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }