Fix 'enumeration not handled in switch' error on Win-Clang bots

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 f5a6e043e0246c9bc6b59830e02094bb5f64cf24..0f1937952b3112526b42d02c7d90f5736528c02e 100644
--- a/ipc/attachment_broker_unprivileged_win.cc
+++ b/ipc/attachment_broker_unprivileged_win.cc
@@ -28,6 +28,7 @@ bool AttachmentBrokerUnprivilegedWin::SendAttachmentToProcess(
       return get_sender()->Send(
           new AttachmentBrokerMsg_DuplicateWinHandle(format));
     }
+    case BrokerableAttachment::MACH_PORT:
     case BrokerableAttachment::PLACEHOLDER:
       NOTREACHED();
       return false;