gfx::GpuMemoryBufferHandle owns gfx::NativePixmapHandle, which, in ozone, holds a list of base::FileDescriptor objects. Serializing base::FileDescriptor for mojo is a little bit tricky, since the serialization code runs twice; and it is necessary to ensure (by way of having a SetUpContext for the struct-traits) that the serialization happens just once.

It may be possible to have NativePixmapHandle use ScopedFD instead to retain ownership of the fds. But then, it would be necessary to make NativePixmapHandle, GpuMemoryBufferHandle etc. be move-only types.