They were passed as size_t, while GL level indices are GLint throughout
ANGLE.

Bug: angleproject:4880
Change-Id: I43bc9f01a607e457b55a061f473450d3360db808
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2380230
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>