This matches the types used by GL and stored in the state.  setStorage
and setStorageMultisampled calls were using size_t instead with a number
of unnecessary casts in places which are now removed.

Bug: angleproject:4836
Change-Id: Ibb570f0c088a6f29abe10f513de27c30ba4dd098
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2411697Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>