Tests ScalePlane vs ScalePlane_16 match.

Bug: libyuv:749
Test: LibYUVScaleTest.ScalePlaneDownBy4_Box_16
Change-Id: I3f71748da404982d5d48bfb11bbd3ae95a1d021c
Reviewed-on: https://chromium-review.googlesource.com/765045Reviewed-by: Frank Barchard <fbarchard@google.com>
Reviewed-by: richard winterton <rrwinterton@gmail.com>
Reviewed-by: Weiyong Yao <braveyao@chromium.org>