Revert of Remove IPC::BrokerableAttachment.

ipc/mach_port_mac.cc

Index: ipc/mach_port_mac.cc
diff --git a/ipc/mach_port_mac.cc b/ipc/mach_port_mac.cc
index 6d3045a560660f139a611da4d85ede2addb99bd1..a482d24d7f03ee1d4de77b773bbf89fe43f00b9a 100644
--- a/ipc/mach_port_mac.cc
+++ b/ipc/mach_port_mac.cc
@@ -34,10 +34,16 @@
     return false;
   MessageAttachment* attachment =
       static_cast<MessageAttachment*>(base_attachment.get());
-  if (attachment->GetType() != MessageAttachment::Type::MACH_PORT)
+  if (attachment->GetType() != MessageAttachment::TYPE_BROKERABLE_ATTACHMENT)
     return false;
+  BrokerableAttachment* brokerable_attachment =
+      static_cast<BrokerableAttachment*>(attachment);
+  if (brokerable_attachment->GetBrokerableType() !=
+      BrokerableAttachment::MACH_PORT) {
+    return false;
+  }
   IPC::internal::MachPortAttachmentMac* mach_port_attachment =
-      static_cast<IPC::internal::MachPortAttachmentMac*>(attachment);
+      static_cast<IPC::internal::MachPortAttachmentMac*>(brokerable_attachment);
   r->set_mach_port(mach_port_attachment->get_mach_port());
   mach_port_attachment->reset_mach_port_ownership();
   return true;