Revert of Remove IPC::BrokerableAttachment.

ipc/ipc_channel_mojo_unittest.cc

Index: ipc/ipc_channel_mojo_unittest.cc
diff --git a/ipc/ipc_channel_mojo_unittest.cc b/ipc/ipc_channel_mojo_unittest.cc
index 5c23ea3823c6b407c7fbf32d16354913dc42e314..229716edc0dbae08db8694e8537745961347d0bc 100644
--- a/ipc/ipc_channel_mojo_unittest.cc
+++ b/ipc/ipc_channel_mojo_unittest.cc
@@ -289,12 +289,11 @@
     base::ScopedFD fd;
     scoped_refptr<base::Pickle::Attachment> attachment;
     EXPECT_TRUE(message.ReadAttachment(iter, &attachment));
-    EXPECT_EQ(
-        IPC::MessageAttachment::Type::PLATFORM_FILE,
-        static_cast<IPC::MessageAttachment*>(attachment.get())->GetType());
-    base::File file(
-        static_cast<IPC::internal::PlatformFileAttachment*>(attachment.get())
-            ->TakePlatformFile());
+    EXPECT_EQ(IPC::MessageAttachment::TYPE_PLATFORM_FILE,
+              static_cast<IPC::MessageAttachment*>(attachment.get())
+                  ->GetType());
+    base::File file(static_cast<IPC::MessageAttachment*>(attachment.get())
+                        ->TakePlatformFile());
     std::string content(GetSendingFileContent().size(), ' ');
     file.Read(0, &content[0], content.size());
     EXPECT_EQ(content, GetSendingFileContent());