NaCl: Address some TODOs from moving PPAPI IRT interface into the Chromium repo

ppapi/proxy/irt_ppapi.c

 /*
  * 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.
  */
 
 #include "native_client/src/public/irt_core.h"
 #include "native_client/src/untrusted/irt/irt.h"
 #include "native_client/src/untrusted/irt/irt_private.h"
 #include "ppapi/nacl_irt/irt_ppapi.h"
@@ skipping 15 matching lines @@
 }
 
 void PPP_ShutdownModule(void) {
   g_pp_functions.PPP_ShutdownModule();
 }
 
 const void* PPP_GetInterface(const char* interface_name) {
   return g_pp_functions.PPP_GetInterface(interface_name);
 }
 
-/*
- * TODO(mseaborn): This can be made static when the NaCl side no
- * longer refers to it.
- */
-const struct nacl_irt_ppapihook nacl_irt_ppapihook = {
+static const struct nacl_irt_ppapihook nacl_irt_ppapihook = {
   irt_ppapi_start,
   PpapiPluginRegisterThreadCreator,
 };
 
 static const struct nacl_irt_interface irt_interfaces[] = {
   { NACL_IRT_PPAPIHOOK_v0_1, &nacl_irt_ppapihook, sizeof(nacl_irt_ppapihook),
     NULL },
 };
 
 static size_t chrome_irt_query(const char* interface_ident,
                                void* table, size_t tablesize) {
   size_t result = nacl_irt_query_core(interface_ident, table, tablesize);
   if (result != 0)
     return result;
   return nacl_irt_query_list(interface_ident, table, tablesize,
                              irt_interfaces, sizeof(irt_interfaces));
 }
 
 void nacl_irt_start(uint32_t* info) {
   nacl_irt_init(info);
   nacl_irt_enter_user_code(info, chrome_irt_query);
 }