The patch updates the cubemap completeness check to use the base level
instead of always level 0. Without this change the cubemap faces at
level 0 would have to be created in order to specify another base level
through TexParameteri.

BUG=angleproject:2153
TEST=angle_end2end_tests

Change-Id: Iee1fdc6adf0e69d797821a2ce2f2b2b85dfcdfc1
Reviewed-on: https://chromium-review.googlesource.com/668439
Commit-Queue: Martin Radev <mradev@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>