Revert "Remove texture sampling from GrConfigConversionEffect"

This reverts commit 3e7cddaf32e280fe9f32eec5bfdd8168ca4941b6.

Reason for revert: Precision issue on some mobile GPUs.

Original change's description:
> Remove texture sampling from GrConfigConversionEffect
> 
> Re-land fixed version of https://skia-review.googlesource.com/c/10026/
> 
> BUG=skia:
> 
> Change-Id: I0754ffb72da2966eb57e5cd7ec818b1cdce84a74
> Reviewed-on: https://skia-review.googlesource.com/10056
> Reviewed-by: Brian Salomon <bsalomon@google.com>
> Commit-Queue: Brian Osman <brianosman@google.com>
> 

TBR=bsalomon@google.com,robertphillips@google.com,brianosman@google.com,reviews@skia.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Change-Id: I66f52efe191b170612775e26d84a2af3d3f856af
Reviewed-on: https://skia-review.googlesource.com/10118
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
3 files changed