H420/H422 are bt.720 variants

TBR=braveyao@chromium.org
BUG=libyuv:799
TESTED=try bots tested build on all platforms

Change-Id: I007d8981d91ca0748c59403759109bbcd88f286c
Reviewed-on: https://chromium-review.googlesource.com/1115719
Commit-Queue: Frank Barchard <fbarchard@chromium.org>
Reviewed-by: Frank Barchard <fbarchard@chromium.org>