Clean update ETC1 data utilities

Change-Id: I03d5d86c617f2f9cca5c1198d242ce0fbb5b2f14
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/217120
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
6 files changed