Revert of Remove IPC::BrokerableAttachment.

ipc/mach_port_attachment_mac.h

Index: ipc/mach_port_attachment_mac.h
diff --git a/ipc/mach_port_attachment_mac.h b/ipc/mach_port_attachment_mac.h
index 861c4e3065635191d42876ff994acb2e17bf6ff7..efc2c11a784a74f8af82fdadd1a467cc97d63a88 100644
--- a/ipc/mach_port_attachment_mac.h
+++ b/ipc/mach_port_attachment_mac.h
@@ -10,16 +10,36 @@
 
 #include "base/macros.h"
 #include "base/process/process_handle.h"
+#include "ipc/brokerable_attachment.h"
 #include "ipc/ipc_export.h"
-#include "ipc/ipc_message_attachment.h"
 #include "ipc/mach_port_mac.h"
 
 namespace IPC {
 namespace internal {
 
 // This class represents an OSX mach_port_t attached to a Chrome IPC message.
-class IPC_EXPORT MachPortAttachmentMac : public MessageAttachment {
+class IPC_EXPORT MachPortAttachmentMac : public BrokerableAttachment {
  public:
+  struct IPC_EXPORT WireFormat {
+    // IPC translation requires that classes passed through IPC have a default
+    // constructor.
+    WireFormat() : mach_port(0), destination_process(0) {}
+
+    WireFormat(uint32_t mach_port, const base::ProcessId& destination_process)
+        : mach_port(mach_port), destination_process(destination_process) {}
+
+    // The mach port that is intended for duplication, or the mach port that has
+    // been duplicated, depending on context.
+    // The type is uint32_t instead of mach_port_t to ensure that the wire
+    // format stays consistent.
+    uint32_t mach_port;
+    static_assert(sizeof(mach_port_t) <= sizeof(uint32_t),
+                  "mach_port_t must be smaller than uint32_t");
+
+    // The id of the destination process that the handle is duplicated into.
+    base::ProcessId destination_process;
+  };
+
   // This constructor increments the ref count of |mach_port_| and takes
   // ownership of the result. Should only be called by the sender of a Chrome
   // IPC message.
@@ -32,7 +52,15 @@
   // ref count. Should only be called by the receiver of a Chrome IPC message.
   MachPortAttachmentMac(mach_port_t mach_port, FromWire from_wire);
 
-  Type GetType() const override;
+  // This constructor takes ownership of |wire_format.mach_port|, but does not
+  // modify its ref count. Should only be called by the receiver of a Chrome IPC
+  // message.
+  explicit MachPortAttachmentMac(const WireFormat& wire_format);
+
+  BrokerableType GetBrokerableType() const override;
+
+  // Returns the wire format of this attachment.
+  WireFormat GetWireFormat(const base::ProcessId& destination) const;
 
   mach_port_t get_mach_port() const { return mach_port_; }