Revert "explictly fail to draw unpremul, as this is buggy, incomplete, tricky, and unused"

This reverts commit 1baaacac74d24604e3dfca9857fbd647df375997.

Reason for revert: Failing the roll: https://storage.googleapis.com/chromium-layout-test-archives/linux_trusty_blink_rel/3868/layout-test-results/results.html

Original change's description:
> explictly fail to draw unpremul, as this is buggy, incomplete, tricky, and unused
> 
> BUG=skia:
> 
> Change-Id: Ie9da6015cc4955c0f27f6db53fc6ae532e0132f4
> Reviewed-on: https://skia-review.googlesource.com/7304
> Commit-Queue: Mike Reed <reed@google.com>
> Reviewed-by: Matt Sarett <msarett@google.com>
> 

TBR=bsalomon@google.com,msarett@google.com,brianosman@google.com,reed@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Change-Id: Ifc808a430b8afe4fef6df3775c2bfdcede1a2dd7
Reviewed-on: https://skia-review.googlesource.com/7329
Commit-Queue: Joe Gregorio <jcgregorio@google.com>
Reviewed-by: Joe Gregorio <jcgregorio@google.com>
4 files changed