Tests accuracy of H010ToAR30 on grey scale ramp against float
and computes a histogram to detect number of unique shades for
each channel.

With 2 step intermediate using 8 bit RGB, the test shows 256
unique values.

[ RUN      ] LibYUVConvertTest.TestH420ToARGB
uniques: B 220, G, 220, R 220
[       OK ] LibYUVConvertTest.TestH420ToARGB (0 ms)
[ RUN      ] LibYUVConvertTest.TestH010ToARGB
uniques: B 256, G, 256, R 256
[       OK ] LibYUVConvertTest.TestH010ToARGB (0 ms)
[ RUN      ] LibYUVConvertTest.TestH010ToAR30
uniques: B 256, G, 256, R 256
[       OK ] LibYUVConvertTest.TestH010ToAR30 (0 ms)

Bug: libyuv:751
Test  LibYUVConvertTest.TestH010ToAR30 unittest

Change-Id: I6b1e1209247cb00b79b594127b02dae5217dc400
Reviewed-on: https://chromium-review.googlesource.com/875317Reviewed-by: Miguel Casas <mcasas@chromium.org>
Commit-Queue: Frank Barchard <fbarchard@chromium.org>