Revert of Remove IPC::BrokerableAttachment.

ipc/ipc_platform_file_attachment_posix.cc

Index: ipc/ipc_platform_file_attachment_posix.cc
diff --git a/ipc/ipc_platform_file_attachment_posix.cc b/ipc/ipc_platform_file_attachment_posix.cc
index 7111cfa0bbb4979d7ff3eb67848c94ccc925baf5..b130ab26eb7db62a4970b2a93ca4237915d7a339 100644
--- a/ipc/ipc_platform_file_attachment_posix.cc
+++ b/ipc/ipc_platform_file_attachment_posix.cc
@@ -20,7 +20,7 @@
 }
 
 MessageAttachment::Type PlatformFileAttachment::GetType() const {
-  return Type::PLATFORM_FILE;
+  return TYPE_PLATFORM_FILE;
 }
 
 base::PlatformFile PlatformFileAttachment::TakePlatformFile() {
@@ -30,7 +30,7 @@
 
 base::PlatformFile GetPlatformFile(
     scoped_refptr<MessageAttachment> attachment) {
-  DCHECK_EQ(attachment->GetType(), MessageAttachment::Type::PLATFORM_FILE);
+  DCHECK_EQ(attachment->GetType(), MessageAttachment::TYPE_PLATFORM_FILE);
   return static_cast<PlatformFileAttachment*>(attachment.get())->file();
 }