ipc: Make a new class PlaceholderBrokerableAttachment.

ipc/attachment_broker_privileged_win.cc

Index: ipc/attachment_broker_privileged_win.cc
diff --git a/ipc/attachment_broker_privileged_win.cc b/ipc/attachment_broker_privileged_win.cc
index c8cc1d0aaf903b5926f20e954e0a823883ca4dfa..20eb4f3beaaadb2ab31825dce2603f6a26b17d5f 100644
--- a/ipc/attachment_broker_privileged_win.cc
+++ b/ipc/attachment_broker_privileged_win.cc
@@ -22,7 +22,7 @@ bool AttachmentBrokerPrivilegedWin::SendAttachmentToProcess(
     const BrokerableAttachment* attachment,
     base::ProcessId destination_process) {
   switch (attachment->GetBrokerableType()) {
-    case BrokerableAttachment::WIN_HANDLE:
+    case BrokerableAttachment::WIN_HANDLE: {
       const internal::HandleAttachmentWin* handle_attachment =
           static_cast<const internal::HandleAttachmentWin*>(attachment);
       HandleWireFormat wire_format =
@@ -33,6 +33,10 @@ bool AttachmentBrokerPrivilegedWin::SendAttachmentToProcess(
         return false;
       RouteDuplicatedHandle(new_wire_format);
       return true;
+    }
+    case BrokerableAttachment::PLACEHOLDER:
+      NOTREACHED();
+      return false;
   }
   return false;
 }