Use string_view inputs for conversion functions (flutter#26031)

This offers a small improvement in performance in cases where the input
is a raw char* or wchar_t* string due to not having to perform an
implicit conversion to std::string/std::wstring.

Code cleanup covered by existing tests, which already use raw C strings.

Author: cbracken

shell/platform/windows/string_conversion.cc

@@ -8,7 +8,7 @@
 
 namespace flutter {
 
-std::string Utf8FromUtf16(const std::wstring& utf16_string) {
+std::string Utf8FromUtf16(const std::wstring_view utf16_string) {
   if (utf16_string.empty()) {
     return std::string();
   }
@@ -30,7 +30,7 @@ std::string Utf8FromUtf16(const std::wstring& utf16_string) {
   return utf8_string;
 }
 
-std::wstring Utf16FromUtf8(const std::string& utf8_string) {
+std::wstring Utf16FromUtf8(const std::string_view utf8_string) {
   if (utf8_string.empty()) {
     return std::wstring();
   }

shell/platform/windows/string_conversion.h

@@ -11,11 +11,11 @@ namespace flutter {
 
 // Converts a string from UTF-16 to UTF-8. Returns an empty string if the
 // input is not valid UTF-16.
-std::string Utf8FromUtf16(const std::wstring& utf16_string);
+std::string Utf8FromUtf16(const std::wstring_view utf16_string);
 
 // Converts a string from UTF-8 to UTF-16. Returns an empty string if the
 // input is not valid UTF-8.
-std::wstring Utf16FromUtf8(const std::string& utf8_string);
+std::wstring Utf16FromUtf8(const std::string_view utf8_string);
 
 }  // namespace flutter