clang/win: Take another stab at building with -Wunused-private-field.

third_party/libphonenumber/libphonenumber.gyp

Index: third_party/libphonenumber/libphonenumber.gyp
diff --git a/third_party/libphonenumber/libphonenumber.gyp b/third_party/libphonenumber/libphonenumber.gyp
index f5a98fe8f9afa751f443003c0ea135da64aee486..4c6b06b6c959af101945a791a506c5ad4c5a3944 100644
--- a/third_party/libphonenumber/libphonenumber.gyp
+++ b/third_party/libphonenumber/libphonenumber.gyp
@@ -66,6 +66,10 @@
     'variables': {
       'proto_in_dir': 'src/resources',
       'proto_out_dir': 'third_party/libphonenumber/phonenumbers',
+      'clang_warning_flags': [
+        # https://github.com/googlei18n/libphonenumber/pull/741
+        '-Wno-unused-private-field',
+      ],
     },
     'includes': [ '../../build/protoc.gypi' ],
     'direct_dependent_settings': {