TBR=kjellander@chromium.org
BUG=libyuv:732
TEST=LibYUVPlanarTest.TestScaleSumSamples_Opt

Change-Id: If80e9510ad5668b080b9384e656c0bd73cf5b4a6
Reviewed-on: https://chromium-review.googlesource.com/663764
Commit-Queue: Frank Barchard <fbarchard@google.com>
Reviewed-by: Frank Barchard <fbarchard@google.com>
Reviewed-by: Cheng Wang <wangcheng@google.com>