Reland "GPU support for SkMixers"

This reverts commit e6ebe7427a0e0d6a42d3d7ef1b2575c0d29c96e1.

Reason for revert: want to see the breaks

Original change's description:
> Revert "GPU support for SkMixers"
> 
> This reverts commit 3f2d05e7af2f0774f987f8fdf10c7c0c05079f5b.
> 
> Reason for revert: breaking the bots
> 
> Original change's description:
> > GPU support for SkMixers
> > 
> > Bug: skia:
> > Change-Id: Ic92f0b3c4613b3d599d59924d93cd4da8d161e89
> > Reviewed-on: https://skia-review.googlesource.com/c/skia/+/204445
> > Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
> > Reviewed-by: Mike Reed <reed@google.com>
> > Reviewed-by: Brian Salomon <bsalomon@google.com>
> 
> TBR=bsalomon@google.com,ethannicholas@google.com,reed@google.com
> 
> Change-Id: Ice7c470a57a3ae55215288718aa6390592bd85e5
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: skia:
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/204724
> Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
> Commit-Queue: Ethan Nicholas <ethannicholas@google.com>

TBR=bsalomon@google.com,ethannicholas@google.com,reed@google.com

Change-Id: If51303bc320fc488196ef9e696df8480bfbd31f6
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: skia:
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/204962
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
14 files changed