BUG=libyuv:717
TEST=ScaleSum unittest to compare C vs Arm implementation
TBR=kjellander@chromium.org

Change-Id: Iaa7af5547d979aad4722f868d31b405340115748
Reviewed-on: https://chromium-review.googlesource.com/600534Reviewed-by: Cheng Wang <wangcheng@google.com>