Change String::WriteUtf8() to return characters written.

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index f64b3867367ab1f65729b04c4b6666c1a385fc1c..90c43831f4165505a8a92c8916a34d255ebabc16 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -855,12 +855,15 @@ class V8EXPORT String : public Primitive {
    * \param start The starting position within the string at which
    * copying begins.
    * \param length The number of bytes to copy from the string.
-   * \return The number of characters copied to the buffer
+   * \param nchars The number of characters written.
+   * \return The number of bytes copied to the buffer
    * excluding the NULL terminator.
    */
   int Write(uint16_t* buffer, int start = 0, int length = -1) const;  // UTF-16
   int WriteAscii(char* buffer, int start = 0, int length = -1) const;  // ASCII
-  int WriteUtf8(char* buffer, int length = -1) const; // UTF-8
+  int WriteUtf8(char* buffer,
+                int length = -1,
+                int* nchars = NULL) const; // UTF-8
 
   /**
    * A zero length string.