Clarify expected compatibility matrix in CharacterizationFBO0nessTest

Change-Id: I120b5f79588f7300c1ac7e9e699cf2423030b328
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/275338
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
1 file changed