Add GM to test ETC1

I don't think ETC1 currently gets tested any other way.

Change-Id: I125d6cb52603c87fc91b47d02743dc4efe0409b7
Reviewed-on: https://skia-review.googlesource.com/8316
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
2 files changed