Let SetEntropySource() fail if called after V8::Initialize().

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 4522 matching lines @@
   /**
    * Initializes from snapshot if possible. Otherwise, attempts to
    * initialize from scratch.  This function is called implicitly if
    * you use the API without calling it first.
    */
   static bool Initialize();
 
   /**
    * Allows the host application to provide a callback which can be used
    * as a source of entropy for random number generators.
+   *
+   * \note Setting an entropy source can only be done very early prior to
+   *   calling Initialize().
    */
   static void SetEntropySource(EntropySource source);
 
   /**
    * Allows the host application to provide a callback that allows v8 to
    * cooperate with a profiler that rewrites return addresses on stack.
    */
   static void SetReturnAddressLocationResolver(
       ReturnAddressLocationResolver return_address_resolver);
 
@@ skipping 1924 matching lines @@
  */
 
 
 }  // namespace v8
 
 
 #undef TYPE_CHECK
 
 
 #endif  // V8_H_