Make UTF-8 encoding of unpaired surrogates match Encoding standard

Source/wtf/text/TextCodecUTF8.cpp

Index: Source/wtf/text/TextCodecUTF8.cpp
diff --git a/Source/wtf/text/TextCodecUTF8.cpp b/Source/wtf/text/TextCodecUTF8.cpp
index ce595b34619709488d0e20aa31b955afaeeb08f3..81197c77e534d3d495062ab480909c7f5ec0d210 100644
--- a/Source/wtf/text/TextCodecUTF8.cpp
+++ b/Source/wtf/text/TextCodecUTF8.cpp
@@ -436,6 +436,10 @@ CString TextCodecUTF8::encodeCommon(const CharType* characters, size_t length)
     while (i < length) {
         UChar32 character;
         U16_NEXT(characters, i, length, character);
+        // U16_NEXT will simply emit a surrogate code point if an unmatched surrogate
+        // is encountered; we must convert it to a U+FFFD (REPLACEMENT CHARACTER) here.
+        if (0xD800 <= character && character <= 0xDFFF)
+            character = replacementCharacter;
         U8_APPEND_UNSAFE(bytes.data(), bytesWritten, character);
     }