Only generate valid premul test data in SRGBReadWritePixels test

Change-Id: I9ca830b2bcf3341b06bc4e261dba40809b452194
Reviewed-on: https://skia-review.googlesource.com/140572
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
1 file changed