buffer.h: add num_elements_
buffer.h: add num_elements_

raw_size_ was being incorrectly computed and used

Change-Id: Iade45f69964c567ffb258880f26006a96ae5a30d

webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx