message: remove vestigal abstact_fd
diff: @@ -91,7 +91,6 @@ class FileBackedMmapHeap : public mesh::MmapHeap {
  private:
  DISALLOW_COPY_AND_ASSIGN(FileBackedMmapHeap);
  typedef MmapHeap SuperHeap;
  - typedef int abstract_fd;
  public:
  enum { Alignment = MmapWrapper::Alignment };
  