Revert of Remove IPC::BrokerableAttachment.

ipc/ipc_message_utils.cc

Index: ipc/ipc_message_utils.cc
diff --git a/ipc/ipc_message_utils.cc b/ipc/ipc_message_utils.cc
index 8ec3c188b2a6f1a2fab8adcc6c1707f337cfa917..5a777e169e370870d3f5c7e778396a7ca6358864 100644
--- a/ipc/ipc_message_utils.cc
+++ b/ipc/ipc_message_utils.cc
@@ -657,14 +657,8 @@
   if (!m->ReadAttachment(iter, &attachment))
     return false;
 
-  if (static_cast<MessageAttachment*>(attachment.get())->GetType() !=
-      MessageAttachment::Type::PLATFORM_FILE) {
-    return false;
-  }
-
   *r = base::FileDescriptor(
-      static_cast<internal::PlatformFileAttachment*>(attachment.get())
-          ->TakePlatformFile(),
+      static_cast<MessageAttachment*>(attachment.get())->TakePlatformFile(),
       true);
   return true;
 }