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

build/set_clang_warning_flags.gypi

Index: build/set_clang_warning_flags.gypi
diff --git a/build/set_clang_warning_flags.gypi b/build/set_clang_warning_flags.gypi
index 3fd5530a4edc0dc3768681e3613578b294834944..3fe75d6a6b12f60bdc28f74456b795893aca0ee2 100644
--- a/build/set_clang_warning_flags.gypi
+++ b/build/set_clang_warning_flags.gypi
@@ -39,6 +39,12 @@
         'WARNING_CFLAGS': ['>@(clang_warning_flags)'],
         'WARNING_CFLAGS!': ['>@(clang_warning_flags_unset)'],
       },
+      'msvs_settings': {
+        'VCCLCompilerTool': {
+          'AdditionalOptions': [ '>@(clang_warning_flags)' ],
+          'AdditionalOptions!': [ '>@(clang_warning_flags_unset)' ],
+        },
+      },
     }],
   ],
 }