Convert some unit tests from NullGL to Mock context

Bug: skia:
Change-Id: I6ecfed3f7ee096f85ea4cb4920e1029227d36169
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/203703
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
5 files changed