Only one flag ("reverseRowOrder") from PixelPackState was being used,
so instead of including the entire struct in PackPixelsParams, just
include the one flag.

BUG=angleproject:2718

Change-Id: I32e8b30383d198ecba9bf1719c32dda0938fc969
Reviewed-on: https://chromium-review.googlesource.com/1242210
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@google.com>