Refactor texStorage2D and texStorage3D.

BUG=angleproject:747
Change-Id: Id5ab6bbff5ce6debc84318e28b12683bf8b106b1
Reviewed-on: https://chromium-review.googlesource.com/526371
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>