Make UTF-8 encoding of unpaired surrogates match Encoding standard

The Encoding standard says that unpaired UTF-16 surrogates in JS
strings should be converted into U+FFFD (replacement character)
during encode operations. This is (optionally) done already in
WTFString::utf8() but not handled in TextCodecUTF8.

BUG=277035

Review URL: https://chromiumcodereview.appspot.com/23601010

git-svn-id: svn://svn.chromium.org/blink/trunk@157459 bbb929c8-8fbe-4397-9dbb-9b2b20218538