Add more const qualifiers in TextureStorage11 This makes the code easier to follow. BUG=angleproject:1244 TEST=angle_end2end_tests Change-Id: I7787b04217a6771629e69f6f67586f62ce5cfccf Reviewed-on: https://chromium-review.googlesource.com/327053 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>