Fix PSNRHVS computation
Fix PSNRHVS computation

Cherry-pick libaom #1362e15990eccab8101305be418528824fb32245 to fix
PSNRHVS calculation.

BUG=webm:1620

Change-Id: Ife7bd8056fcaed06992ad76e8c1ab734c3956ad7

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