Add api call to determine whether a string can be externalized.

include/v8.h

 // Copyright 2007-2008 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 883 matching lines @@
   /**
    * Associate an external string resource with this string by transforming it
    * in place so that existing references to this string in the JavaScript heap
    * will use the external string resource. The external string resource's
    * character contents needs to be equivalent to this string.
    * Returns true if the string has been changed to be an external string.
    * The string is not modified if the operation fails.
    */
   bool MakeExternal(ExternalAsciiStringResource* resource);
 
+  /**
+   * Returns true if this string can be made external.
+   */
+  bool CanMakeExternal();
+
   /** Creates an undetectable string from the supplied ascii or utf-8 data.*/
   static Local<String> NewUndetectable(const char* data, int length = -1);
 
   /** Creates an undetectable string from the supplied utf-16 data.*/
   static Local<String> NewUndetectable(const uint16_t* data, int length = -1);
 
   /**
    * Converts an object to a utf8-encoded character array.  Useful if
    * you want to print the object.  If conversion to a string fails
    * (eg. due to an exception in the toString() method of the object)
@@ skipping 1761 matching lines @@
 
 }  // namespace v8
 
 
 #undef V8EXPORT
 #undef V8EXPORT_INLINE
 #undef TYPE_CHECK
 
 
 #endif  // V8_H_