use variadic macros/templates in IPC message implementation

ipc/attachment_broker_messages.h

Index: ipc/attachment_broker_messages.h
diff --git a/ipc/attachment_broker_messages.h b/ipc/attachment_broker_messages.h
index a6301c2da134f18be82d5563b5e8f2a54be774ba..fcb76febfb4b6b331ef8960c24d49ac6d31429c0 100644
--- a/ipc/attachment_broker_messages.h
+++ b/ipc/attachment_broker_messages.h
@@ -18,6 +18,10 @@
 #include "ipc/mach_port_attachment_mac.h"
 #endif  // defined(OS_MACOSX)
 
+#undef IPC_MESSAGE_EXPORT

[danakj, 2015/12/19 00:15:06]

Why is this moved?

[mdempsky, 2015/12/19 02:14:48]

I was trying to fix the Windows component build and thought this was the
problem, though this didn't fix it (at least in isolation).  I'll check if this
can be reverted.

(Context: The IPC_STRUCTS_TRAITS_BEGIN macros define template specializations
using IPC_MESSAGE_EXPORT.  At one point I had Windows component build errors
relating to the messages below (e.g., AttachmentBrokerMsg_DuplicateWinHandle)
referring to PC::internal::HandleAttachmentWin::WireFormat's Traits.  I thought
the problem might have been that the traits specializations and messages were
using different EXPORT settings, so I moved this up so they'd have the same.)

+#define IPC_MESSAGE_EXPORT IPC_EXPORT
+#define IPC_MESSAGE_START AttachmentBrokerMsgStart
+
 // ----------------------------------------------------------------------------
 // Serialization of structs.
 // ----------------------------------------------------------------------------
@@ -42,10 +46,6 @@ IPC_STRUCT_TRAITS_BEGIN(IPC::internal::MachPortAttachmentMac::WireFormat)
 IPC_STRUCT_TRAITS_END()
 #endif  // defined(OS_MACOSX)
 
-#undef IPC_MESSAGE_EXPORT
-#define IPC_MESSAGE_EXPORT IPC_EXPORT
-#define IPC_MESSAGE_START AttachmentBrokerMsgStart
-
 // ----------------------------------------------------------------------------
 // Messages sent from the broker to a non-broker process.
 // ----------------------------------------------------------------------------