Revert "Allow linear 8888 surface contexts in GPU backend."

This reverts commit 6a46fb210b810553b2a03a5f316109ccd9710ebc.

Reason for revert: breaks gms

Original change's description:
> 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>

TBR=bsalomon@google.com,brianosman@google.com

Change-Id: Ifb28fdde315ccc0b6fc1c9dd944ea319bafba754
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: skia:
Reviewed-on: https://skia-review.googlesource.com/107004
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
2 files changed