Pack buffers used in different binding points when combined
with other buffer operations require us to be able to read
from the pack buffer then write to the native buffer.

BUG=angle:511

Change-Id: I2e859695235eb649ddd5dfc1332ecb19b57c1933
Reviewed-on: https://chromium-review.googlesource.com/198434Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>