Add support for default values of IPC_STRUCT_MEMBER.

ipc/ipc_message_macros.h

 // Copyright (c) 2012 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 184 matching lines @@
 // 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();
-#define IPC_STRUCT_MEMBER(type, name) type name;
+// Optional variadic parameters specify the default value for this struct
+// member. They are passed through to the constructor for |type|.
+#define IPC_STRUCT_MEMBER(type, name, ...) type name;
 #define IPC_STRUCT_END() };
 
 // Message macros collect specific numbers of arguments and funnel them into
 // the common message generation macro.  These should never be redefined.
 #define IPC_MESSAGE_CONTROL0(msg_class) \
   IPC_MESSAGE_DECL(EMPTY, CONTROL, msg_class, 0, 0, (), ())
 
 #define IPC_MESSAGE_CONTROL1(msg_class, type1) \
   IPC_MESSAGE_DECL(ASYNC, CONTROL, msg_class, 1, 0, (type1), ())
 
@@ skipping 792 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