Revert of Remove IPC::BrokerableAttachment.

ipc/ipc_mojo_handle_attachment.cc

Index: ipc/ipc_mojo_handle_attachment.cc
diff --git a/ipc/ipc_mojo_handle_attachment.cc b/ipc/ipc_mojo_handle_attachment.cc
index e3421c3e88bc31aa78b4d0b51427cb4948bb5d36..819a12b890ad1c3da3857f3f36923c16854576e4 100644
--- a/ipc/ipc_mojo_handle_attachment.cc
+++ b/ipc/ipc_mojo_handle_attachment.cc
@@ -18,8 +18,15 @@
 }
 
 MessageAttachment::Type MojoHandleAttachment::GetType() const {
-  return Type::MOJO_HANDLE;
+  return TYPE_MOJO_HANDLE;
 }
+
+#if defined(OS_POSIX)
+base::PlatformFile MojoHandleAttachment::TakePlatformFile() {
+  NOTREACHED();
+  return base::kInvalidPlatformFile;
+}
+#endif  // OS_POSIX
 
 mojo::ScopedHandle MojoHandleAttachment::TakeHandle() {
   return std::move(handle_);