Allow linear 8888 surface contexts in GPU backend.

Make SRGBReadWritePixels test test all combinations of sRGB, untagged, and linear.

Bug: skia:
Change-Id: I0c75fa27b1bf60c6e7ce3b666ff79e1ad1c91b94
Reviewed-on: https://skia-review.googlesource.com/106922
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
2 files changed