TBR=kjellander@chromium.org
BUG=libyuv:717
TEST=LibYUVPlanarTest.TestScaleSumSamples_Opt

Change-Id: I5208666f3968c5c4b0f1b0c951f24216d78ee3fe
Reviewed-on: https://chromium-review.googlesource.com/607184Reviewed-by: Cheng Wang <wangcheng@google.com>