For more complete support of AR30 format, add I420ToAR30 allowing
the new RGB 10 bit format to be used from standard 8 bit I420 format.

Bug: libyuv:751
Test: I420ToAR30 unittest added
Change-Id: Ia8b0857447408bd6adab485158ce5f38d6dc2faa
Reviewed-on: https://chromium-review.googlesource.com/823243Reviewed-by: Weiyong Yao <braveyao@chromium.org>
Commit-Queue: Frank Barchard <fbarchard@chromium.org>