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

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index c5a8facb9e1122e7fce09a5cdfaca06c486d7047..9ace0bbefb9c7cd36e92773ee6e762b2c7965d12 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -2168,17 +2168,23 @@
                 ],
               },
               '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,