IPC: Remove unnecessary conversions of BrokerableAttachment.

ipc/attachment_broker_unprivileged_mac.h

Index: ipc/attachment_broker_unprivileged_mac.h
diff --git a/ipc/attachment_broker_unprivileged_mac.h b/ipc/attachment_broker_unprivileged_mac.h
index 26cf4437b7ae6f5b814438e3bfadcd73f5ad0bd6..b47233f689fbf89c15cf9fcc3e1d618e2771b113 100644
--- a/ipc/attachment_broker_unprivileged_mac.h
+++ b/ipc/attachment_broker_unprivileged_mac.h
@@ -22,8 +22,9 @@ class IPC_EXPORT AttachmentBrokerUnprivilegedMac
   ~AttachmentBrokerUnprivilegedMac() override;
 
   // IPC::AttachmentBroker overrides.
-  bool SendAttachmentToProcess(BrokerableAttachment* attachment,
-                               base::ProcessId destination_process) override;
+  bool SendAttachmentToProcess(
+      const scoped_refptr<IPC::BrokerableAttachment>& attachment,
+      base::ProcessId destination_process) override;
 
   // IPC::Listener overrides.
   bool OnMessageReceived(const Message& message) override;