String:WriteUtf8: Add REPLACE_INVALID_UTF8 option

test/cctest/test-parsing.cc

Index: test/cctest/test-parsing.cc
diff --git a/test/cctest/test-parsing.cc b/test/cctest/test-parsing.cc
index e53250def93322a9e965ac0b2be1455856375e47..bb77b94ccfc9aa87fc93bece05bdd31e3278d1c5 100644
--- a/test/cctest/test-parsing.cc
+++ b/test/cctest/test-parsing.cc
@@ -578,6 +578,7 @@ TEST(Utf8CharacterStream) {
   char buffer[kAllUtf8CharsSizeU];
   unsigned cursor = 0;
   for (int i = 0; i <= kMaxUC16Char; i++) {
+    // @TODO Fix this Utf8::Encode call site somehow.
     cursor += unibrow::Utf8::Encode(buffer + cursor,
                                     i,
                                     unibrow::Utf16::kNoPreviousCharacter);