ipc: Make a new class PlaceholderBrokerableAttachment.

ipc/attachment_broker_unprivileged_win.cc

Index: ipc/attachment_broker_unprivileged_win.cc
diff --git a/ipc/attachment_broker_unprivileged_win.cc b/ipc/attachment_broker_unprivileged_win.cc
index 3d3e845d6b8d50c68bf611e05a28c58960ce96eb..f5a6e043e0246c9bc6b59830e02094bb5f64cf24 100644
--- a/ipc/attachment_broker_unprivileged_win.cc
+++ b/ipc/attachment_broker_unprivileged_win.cc
@@ -20,13 +20,17 @@ bool AttachmentBrokerUnprivilegedWin::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);
       internal::HandleAttachmentWin::WireFormat format =
           handle_attachment->GetWireFormat(destination_process);
       return get_sender()->Send(
           new AttachmentBrokerMsg_DuplicateWinHandle(format));
+    }
+    case BrokerableAttachment::PLACEHOLDER:
+      NOTREACHED();
+      return false;
   }
   return false;
 }