Make IPC_MESSAGE_EXPORT more robust.

ipc/ipc_message_macros.h

Index: ipc/ipc_message_macros.h
diff --git a/ipc/ipc_message_macros.h b/ipc/ipc_message_macros.h
index 783108eaab4c9a1c702b404b1f5ae20bc8f0d031..071a433a8a3c2846f629bed765aad8c7c53b393f 100644
--- a/ipc/ipc_message_macros.h
+++ b/ipc/ipc_message_macros.h
@@ -173,6 +173,14 @@
 //     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_
 
@@ -184,11 +192,6 @@
 #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)