Downsample 16x2 to 8x1 with box filtering

[ RUN      ] LibYUVScaleTest.TestScaleRowUp2_16
[       OK ] LibYUVScaleTest.TestScaleRowUp2_16 (579 ms)
[ RUN      ] LibYUVScaleTest.TestScaleRowDown2Box_16
[       OK ] LibYUVScaleTest.TestScaleRowDown2Box_16 (329 ms)
[----------] 2 tests from LibYUVScaleTest (909 ms total)

TBR=kjellander@chromium.org
BUG=libyuv:718
TEST=LibYUVScaleTest.TestScaleRowUp2_16 and LibYUVScaleTest.TestScaleRowDown2Box_16

Change-Id: I457d44123f2751e5f71bf3935401fff74b8e9db2
Reviewed-on: https://chromium-review.googlesource.com/608876Reviewed-by: Cheng Wang <wangcheng@google.com>