Fix and consolidate wstring conversion utils (flutter#16342)

There were two variants of string/wstring conversion utils, one using
codecvt_utf8 and the other using codecvt_utf8_utf16. We want the latter,
since we want to be using UTF-16, not UCS2.

fml/platform/win/file_win.cc

@@ -76,7 +76,7 @@ static DWORD GetShareFlags(FilePermission permission) {
 }
 
 static DWORD GetFileAttributesForUtf8Path(const char* absolute_path) {
-  return ::GetFileAttributes(ConvertToWString(absolute_path).c_str());
+  return ::GetFileAttributes(StringToWideString(absolute_path).c_str());
 }
 
 static DWORD GetFileAttributesForUtf8Path(const fml::UniqueFD& base_directory,
@@ -272,7 +272,7 @@ bool IsFile(const std::string& path) {
 }
 
 bool UnlinkDirectory(const char* path) {
-  if (!::RemoveDirectory(ConvertToWString(path).c_str())) {
+  if (!::RemoveDirectory(StringToWideString(path).c_str())) {
     FML_DLOG(ERROR) << "Could not remove directory: '" << path << "'. "
                     << GetLastErrorMessage();
     return false;
@@ -291,7 +291,7 @@ bool UnlinkDirectory(const fml::UniqueFD& base_directory, const char* path) {
 }
 
 bool UnlinkFile(const char* path) {
-  if (!::DeleteFile(ConvertToWString(path).c_str())) {
+  if (!::DeleteFile(StringToWideString(path).c_str())) {
     FML_DLOG(ERROR) << "Could not remove file: '" << path << "'. "
                     << GetLastErrorMessage();
     return false;

fml/platform/win/native_library_win.cc

@@ -16,7 +16,7 @@ NativeLibrary::NativeLibrary(const char* path)
     return;
   }
 
-  handle_ = ::LoadLibrary(ConvertToWString(path).c_str());
+  handle_ = ::LoadLibrary(StringToWideString(path).c_str());
 }
 
 NativeLibrary::NativeLibrary(Handle handle, bool close_handle)

fml/platform/win/wstring_conversion.h

@@ -12,16 +12,7 @@
 namespace fml {
 
 using WideStringConvertor =
-    std::wstring_convert<std::codecvt_utf8<wchar_t>, wchar_t>;
-
-inline std::wstring ConvertToWString(const char* path) {
-  if (path == nullptr) {
-    return {};
-  }
-  std::string path8(path);
-  std::wstring_convert<std::codecvt_utf8_utf16<wchar_t>, wchar_t> wchar_conv;
-  return wchar_conv.from_bytes(path8);
-}
+    std::wstring_convert<std::codecvt_utf8_utf16<wchar_t>, wchar_t>;
 
 inline std::wstring StringToWideString(const std::string& str) {
   WideStringConvertor converter;