add function to test whether string contents are definitely one byte

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index 28a63f092efb5a7831ccd5a891a9cffddbf4f655..a8221078030fa64c5ebe859d4ebe106daa8a4688 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -1524,11 +1524,19 @@ class V8EXPORT String : public Primitive {
   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