H010ToAR30 uses Convert16To8Row_SSSE3 to convert 10 bit YUV to 8 bit.
Then standard YUV conversion can be used.  This improves performance
on low end CPUs.
Future CL will by pass this conversion allowing for 10 bit YUV source,
but the function will be useful as a utility for YUV conversions.

Bug: libyuv:559, libyuv:751
Test: out/Release/libyuv_unittest --gtest_filter=*H010ToAR30* --libyuv_width=1280 --libyuv_height=720 --libyuv_repeat=999 --libyuv_flags=-1 --libyuv_cpu_info=-1
Change-Id: I9b3ef22d88a5fd861de4cf1900b4c6e8fd24d0af
Reviewed-on: https://chromium-review.googlesource.com/792334
Commit-Queue: Frank Barchard <fbarchard@chromium.org>
Reviewed-by: Frank Barchard <fbarchard@chromium.org>