We were reserving half the required descriptor sets for our pool. This
fixes the counting and ensures we won't regress by adding a test.

Also enables the cube map texture sample, and removes an UNIMPLEMENTED
warning that was spurious.

Bug: angleproject:2318

Change-Id: I371cd7c5b42e1ce980cce7bb0ef04885db72b614
Reviewed-on: https://chromium-review.googlesource.com/1014165
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>