clang/win: Let some chromium_code targets build with -Wextra.

base/win/registry_unittest.cc

Index: base/win/registry_unittest.cc
diff --git a/base/win/registry_unittest.cc b/base/win/registry_unittest.cc
index 22576634f9a4aaa6e3906bea3130f98dd89924e8..0b87023c22a5ca4120a2fe2e74f154b87bf0c04c 100644
--- a/base/win/registry_unittest.cc
+++ b/base/win/registry_unittest.cc
@@ -167,7 +167,7 @@ TEST_F(RegistryTest, TruncatedCharTest) {
   const wchar_t kName[] = L"name";
   // kData size is not a multiple of sizeof(wchar_t).
   const uint8 kData[] = { 1, 2, 3, 4, 5 };
-  EXPECT_EQ(5, arraysize(kData));
+  EXPECT_EQ(5u, arraysize(kData));
   ASSERT_EQ(ERROR_SUCCESS, key.WriteValue(kName, kData,
                                           arraysize(kData), REG_BINARY));