String:WriteUtf8: Add REPLACE_INVALID_UTF8 option

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index fbdc82f8f58c9aa945a53e63ce8be00f78d1d0f6..96d8c6fb6a136c54784850ba10fe23489529ec89 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -1628,7 +1628,11 @@ class V8_EXPORT String : public Primitive {
     NO_OPTIONS = 0,
     HINT_MANY_WRITES_EXPECTED = 1,
     NO_NULL_TERMINATION = 2,
-    PRESERVE_ASCII_NULL = 4
+    PRESERVE_ASCII_NULL = 4,
+    // Used by WriteUtf8 to replace orphan surrogate code units with the
+    // unicode replacement character. Needs to be set to guarantee valid UTF-8
+    // output.
+    REPLACE_INVALID_UTF8 = 8
   };
 
   // 16-bit character codes.