Make IPC_MESSAGE_EXPORT more robust.

ipc/ipc_message_macros.h

 // Copyright (c) 2011 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.
 
 // Defining IPC Messages
 //
 // Your IPC messages will be defined by macros inside of an XXX_messages.h
 // header file.  Most of the time, the system can automatically generate all
 // of messaging mechanism from these definitions, but sometimes some manual
 // coding is required.  In these cases, you will also have an XXX_messages.cc
@@ skipping 155 matching lines @@
 //     IPC_MESSAGE_HANDLER_DELAY_REPLY(ViewHostMsg_SyncMessageName,
 //                                     OnSyncMessageName)
 //
 // The handler function will look like:
 //     void OnSyncMessageName(const type1& in1, IPC::Message* reply_msg);
 //
 // Receiver stashes the IPC::Message* pointer, and when it's ready, it does:
 //     ViewHostMsg_SyncMessageName::WriteReplyParams(reply_msg, out1, out2);
 //     Send(reply_msg);
 
+// Files that want to export their ipc messages should do
+//   #undef IPC_MESSAGE_EXPORT
+//   #define IPC_MESSAGE_EXPORT VISIBILITY_MACRO
+// after including this header, but before using any of the macros below.
+// (This needs to be before the include guard.)
+#undef IPC_MESSAGE_EXPORT
+#define IPC_MESSAGE_EXPORT
+
 #ifndef IPC_IPC_MESSAGE_MACROS_H_
 #define IPC_IPC_MESSAGE_MACROS_H_
 
 #include "base/profiler/scoped_profile.h"
 #include "ipc/ipc_message_utils.h"
 #include "ipc/param_traits_macros.h"
 
 #if defined(IPC_MESSAGE_IMPL)
 #include "ipc/ipc_message_utils_impl.h"
 #endif
 
-// Override this to force message classes to be exported.
-#ifndef IPC_MESSAGE_EXPORT
-#define IPC_MESSAGE_EXPORT
-#endif
-
 // Macros for defining structs.  May be subsequently redefined.
 #define IPC_STRUCT_BEGIN(struct_name) \
   IPC_STRUCT_BEGIN_WITH_PARENT(struct_name, IPC::NoParams)
 #define IPC_STRUCT_BEGIN_WITH_PARENT(struct_name, parent) \
   struct struct_name; \
   IPC_STRUCT_TRAITS_BEGIN(struct_name) \
   IPC_STRUCT_TRAITS_END() \
   struct IPC_MESSAGE_EXPORT struct_name : parent { \
     struct_name(); \
     ~struct_name();
@@ skipping 803 matching lines @@
 // This corresponds to an enum value from IPCMessageStart.
 #define IPC_MESSAGE_CLASS(message) \
   IPC_MESSAGE_ID_CLASS(message.type())
 
 #endif  // IPC_IPC_MESSAGE_MACROS_H_
 
 // Clean up IPC_MESSAGE_START in this unguarded section so that the
 // XXX_messages.h files need not do so themselves.  This makes the
 // XXX_messages.h files easier to write.
 #undef IPC_MESSAGE_START