add function to test whether string contents are definitely one byte

include/v8.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1506 matching lines @@
    * representation of this string.
    */
   int Utf8Length() const;
 
   /**
    * This function is no longer useful.
    */
   V8_DEPRECATED(V8_INLINE(bool MayContainNonAscii()) const) { return true; }
 
   /**
-   * Returns whether this string contains only one byte data.
+   * Returns whether this string is known to contain only one byte data.
+   * Does not read the string.
+   * False negatives are possible.
    */
   bool IsOneByte() const;
 
   /**
+   * Returns whether this string contain only one byte data.
+   * Will read the entire string in some cases.
+   */
+  bool ContainsOnlyOneByte() const;
+
+  /**
    * Write the contents of the string to an external buffer.
    * If no arguments are given, expects the buffer to be large
    * enough to hold the entire string and NULL terminator. Copies
    * the contents of the string and the NULL terminator into the
    * buffer.
    *
    * WriteUtf8 will not write partial UTF-8 sequences, preferring to stop
    * before the end of the buffer.
    *
    * Copies up to length characters into the output buffer.
@@ skipping 4734 matching lines @@
 
 
 }  // namespace v8
 
 
 #undef V8EXPORT
 #undef TYPE_CHECK
 
 
 #endif  // V8_H_