This patch introduces IsCubeMapFaceTarget() to determine if a
teture target belongs to cube map.

BUG=angleproject:2169

Change-Id: I3968ee267887665835f3eb3eda281c054e5d4375
Reviewed-on: https://chromium-review.googlesource.com/1080450Reviewed-by: Jiajia Qin <jiajia.qin@intel.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>