IPC: Remove unnecessary conversions of BrokerableAttachment.

ipc/attachment_broker_privileged_mac.cc

Index: ipc/attachment_broker_privileged_mac.cc
diff --git a/ipc/attachment_broker_privileged_mac.cc b/ipc/attachment_broker_privileged_mac.cc
index 61694bf4e2f0820fcff8cc5074af18feb59e72d8..8965eb5ad19f19a3c9f2af7fb8fbe2cc8bd646c5 100644
--- a/ipc/attachment_broker_privileged_mac.cc
+++ b/ipc/attachment_broker_privileged_mac.cc
@@ -60,12 +60,12 @@ AttachmentBrokerPrivilegedMac::AttachmentBrokerPrivilegedMac(
 AttachmentBrokerPrivilegedMac::~AttachmentBrokerPrivilegedMac() {}
 
 bool AttachmentBrokerPrivilegedMac::SendAttachmentToProcess(
-    BrokerableAttachment* attachment,
+    const scoped_refptr<IPC::BrokerableAttachment>& attachment,
     base::ProcessId destination_process) {
   switch (attachment->GetBrokerableType()) {
     case BrokerableAttachment::MACH_PORT: {
       internal::MachPortAttachmentMac* mach_port_attachment =
-          static_cast<internal::MachPortAttachmentMac*>(attachment);
+          static_cast<internal::MachPortAttachmentMac*>(attachment.get());
       MachPortWireFormat wire_format =
           mach_port_attachment->GetWireFormat(destination_process);