Revert "Remove unused unpremul_float"

This reverts commit e7ff4bc9b8b274aafc005d3d136bcfab7e198097.

Reason for revert: breaking tree: http://screen/4HKjJprbCKA87jf

Original change's description:
> Remove unused unpremul_float
>
> Change-Id: I59821d748768d74b251d9339787f193c23e3d4dc
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/358526
> Reviewed-by: John Stiles <johnstiles@google.com>
> Reviewed-by: Brian Osman <brianosman@google.com>
> Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
> Commit-Queue: Brian Osman <brianosman@google.com>
> Auto-Submit: Ethan Nicholas <ethannicholas@google.com>

TBR=brianosman@google.com,ethannicholas@google.com,johnstiles@google.com

Change-Id: I9997ee46b3db3befc54692981cd4bc2be5cee268
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/359116
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
4 files changed