Reland #2 "ipc: Add a new field num_brokered_attachments to the message header."

ipc/ipc_message.h

Index: ipc/ipc_message.h
diff --git a/ipc/ipc_message.h b/ipc/ipc_message.h
index 8210880860fe010d1fd2acaf55519515f2d28a46..af102d7892f1ec1e843bd84b1f3e095f0cd4f4df 100644
--- a/ipc/ipc_message.h
+++ b/ipc/ipc_message.h
@@ -12,6 +12,7 @@
 #include "base/memory/ref_counted.h"
 #include "base/pickle.h"
 #include "base/trace_event/trace_event.h"
+#include "ipc/attachment_broker.h"
 #include "ipc/brokerable_attachment.h"
 #include "ipc/ipc_export.h"
 
@@ -251,6 +252,12 @@ class IPC_EXPORT Message : public base::Pickle {
     int32_t routing;  // ID of the view that this message is destined for
     uint32_t type;    // specifies the user-defined message type
     uint32_t flags;   // specifies control flags for the message
+#if USE_ATTACHMENT_BROKER
+    // The number of brokered attachments included with this message. The
+    // ids of the brokered attachment ids are sent immediately after the pickled
+    // message, before the next pickled message is sent.
+    uint32_t num_brokered_attachments;
+#endif
 #if defined(OS_POSIX)
     uint16_t num_fds; // the number of descriptors included with this message
     uint16_t pad;     // explicitly initialize this to appease valgrind