String:WriteUtf8: Add REPLACE_INVALID_UTF8 option

src/unicode.h

Index: src/unicode.h
diff --git a/src/unicode.h b/src/unicode.h
index 6ba61d0e17b2a0b6ff7702c422c38ddf9276318a..bb5506d38e2531b0f60fd73f8676c3ad77277ab7 100644
--- a/src/unicode.h
+++ b/src/unicode.h
@@ -102,6 +102,9 @@ class UnicodeData {
 
 class Utf16 {
  public:
+  static inline bool IsSurrogatePair(int lead, int trail) {
+    return IsLeadSurrogate(lead) && IsTrailSurrogate(trail);
+  }
   static inline bool IsLeadSurrogate(int code) {
     if (code == kNoPreviousCharacter) return false;
     return (code & 0xfc00) == 0xd800;
@@ -146,11 +149,16 @@ class Utf8 {
  public:
   static inline uchar Length(uchar chr, int previous);
   static inline unsigned EncodeOneByte(char* out, uint8_t c);
-  static inline unsigned Encode(
-      char* out, uchar c, int previous);
+  static inline unsigned Encode(char* out,
+                                uchar c,
+                                int previous,
+                                bool replace_invalid = false);
   static uchar CalculateValue(const byte* str,
                               unsigned length,
                               unsigned* cursor);
+
+  // The unicode replacement character, used to signal invalid unicode
+  // sequences (e.g. an orphan surrogate) when converting to a UTF-8 encoding.
   static const uchar kBadChar = 0xFFFD;
   static const unsigned kMaxEncodedSize   = 4;
   static const unsigned kMaxOneByteChar   = 0x7f;
@@ -162,6 +170,9 @@ class Utf8 {
   // that match are coded as a 4 byte UTF-8 sequence.
   static const unsigned kBytesSavedByCombiningSurrogates = 2;
   static const unsigned kSizeOfUnmatchedSurrogate = 3;
+  // The maximum size a single UTF-16 code unit may take up when encoded as
+  // UTF-8.
+  static const unsigned kMax16BitCodeUnitSize  = 3;
   static inline uchar ValueOf(const byte* str,
                               unsigned length,
                               unsigned* cursor);