Revert of Remove IPC::BrokerableAttachment.

ipc/handle_win.cc

Index: ipc/handle_win.cc
diff --git a/ipc/handle_win.cc b/ipc/handle_win.cc
index c8511e2017387ac523f238f6c8c76dae04f8c90d..60fa54c7cd1643cd08a3ed8093ab7d02d4f4e7b7 100644
--- a/ipc/handle_win.cc
+++ b/ipc/handle_win.cc
@@ -38,10 +38,16 @@
     return false;
   MessageAttachment* attachment =
       static_cast<MessageAttachment*>(base_attachment.get());
-  if (attachment->GetType() != MessageAttachment::Type::WIN_HANDLE)
+  if (attachment->GetType() != MessageAttachment::TYPE_BROKERABLE_ATTACHMENT)
     return false;
+  BrokerableAttachment* brokerable_attachment =
+      static_cast<BrokerableAttachment*>(attachment);
+  if (brokerable_attachment->GetBrokerableType() !=
+      BrokerableAttachment::WIN_HANDLE) {
+    return false;
+  }
   IPC::internal::HandleAttachmentWin* handle_attachment =
-      static_cast<IPC::internal::HandleAttachmentWin*>(attachment);
+      static_cast<IPC::internal::HandleAttachmentWin*>(brokerable_attachment);
   r->set_handle(handle_attachment->get_handle());
   handle_attachment->reset_handle_ownership();
   return true;