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

base/test/test_reg_util_win_unittest.cc

Index: base/test/test_reg_util_win_unittest.cc
diff --git a/base/test/test_reg_util_win_unittest.cc b/base/test/test_reg_util_win_unittest.cc
index 216d58ea11116b40b96f1572315f474cdb5f99a1..c38baa3fc5f5cf5bb3baa01ecfd06d1aecfa9450 100644
--- a/base/test/test_reg_util_win_unittest.cc
+++ b/base/test/test_reg_util_win_unittest.cc
@@ -89,7 +89,7 @@ TEST_F(RegistryOverrideManagerTest, Basic) {
             read_key.Open(HKEY_CURRENT_USER, kTestKeyPath, KEY_READ));
   EXPECT_TRUE(read_key.Valid());
   EXPECT_EQ(ERROR_SUCCESS, read_key.ReadValueDW(kTestValueName, &value));
-  EXPECT_EQ(42, value);
+  EXPECT_EQ(42u, value);
   read_key.Close();
 
   manager_.reset();