Remove usage of deprecated std::codecvt

It was causing issues building Dawn in Skia. The error message helpfully
suggests using WideCharToMultiByte instead.

Bug: dawn:706
Change-Id: I87dee1192123f49cc8e7243a29df446389454df4
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/44560
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Auto-Submit: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Austin Eng <enga@chromium.org>

src/tests/unittests/WindowsUtilsTests.cpp

@@ -0,0 +1,34 @@
+// Copyright 2021 The Dawn Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <gtest/gtest.h>
+
+#include "common/WindowsUtils.h"
+
+TEST(WindowsUtilsTests, WCharToUTF8) {
+    // Test the empty string
+    ASSERT_EQ("", WCharToUTF8(L""));
+
+    // Test ASCII characters
+    ASSERT_EQ("abc", WCharToUTF8(L"abc"));
+
+    // Test ASCII characters
+    ASSERT_EQ("abc", WCharToUTF8(L"abc"));
+
+    // Test two-byte utf8 character
+    ASSERT_EQ("\xd1\x90", WCharToUTF8(L"\x450"));
+
+    // Test three-byte utf8 codepoint
+    ASSERT_EQ("\xe1\x81\x90", WCharToUTF8(L"\x1050"));
+}