NaCl: Move definition of "struct nacl_irt_ppapihook" to the Chromium repo

ppapi/nacl_irt/irt_ppapi.h

 /*
  * Copyright 2014 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
-/*
- * TODO(mseaborn): Change this to match the filename when NaCl's copy
- * of irt_ppapi.h is removed.  For now it is needed to avoid a
- * duplicate definition.
- */
-#ifndef NATIVE_CLIENT_SRC_UNTRUSTED_IRT_IRT_PPAPI_H_
-#define NATIVE_CLIENT_SRC_UNTRUSTED_IRT_IRT_PPAPI_H_ 1
+#ifndef PPAPI_NACL_IRT_IRT_PPAPI_H_
+#define PPAPI_NACL_IRT_IRT_PPAPI_H_
 
 #include <stddef.h>
 
 #include "ppapi/c/ppp.h"
 
 struct PP_StartFunctions {
   int32_t (*PPP_InitializeModule)(PP_Module module_id,
                                   PPB_GetInterface get_browser_interface);
   void (*PPP_ShutdownModule)();
   const void* (*PPP_GetInterface)(const char* interface_name);
 };
 
 struct PP_ThreadFunctions {
   /*
    * This is a cut-down version of pthread_create()/pthread_join().
    * We omit thread creation attributes and the thread's return value.
    *
    * We use uintptr_t as the thread ID type because pthread_t is not
    * part of the stable ABI; a user thread library might choose an
    * arbitrary size for its own pthread_t.
    */
   int (*thread_create)(uintptr_t* tid,
                        void (*func)(void* thread_argument),
                        void* thread_argument);
   int (*thread_join)(uintptr_t tid);
 };
 
+/*
+ * TODO(mseaborn): Make this unconditional after the definition of the
+ * struct has been removed from the NaCl side's irt.h.
+ */
+#if !defined(NACL_IRT_PPAPIHOOK_v0_1)
+
+#define NACL_IRT_PPAPIHOOK_v0_1 "nacl-irt-ppapihook-0.1"
+struct nacl_irt_ppapihook {
+  int (*ppapi_start)(const struct PP_StartFunctions*);
+  void (*ppapi_register_thread_creator)(const struct PP_ThreadFunctions*);
+};
+
 #endif
+
+#endif