Enable legacy premuls in SkColorSpaceXform

***Will allow for simplified Android framework code, they typically
   want a color correct transform followed by a gamma encoded premul.
***Chrome does the same, so this will make it easier to replace their
   codecs.
***Will decrease code size.  Both types of premuls are moved off the
   fast path here - one is essentially unused in production and the
   other is not "encouraged".
***Will actually make the common case faster: sRGB->sRGB means no
   color xform, just premul in SkSwizzler.

BUG=skia:

CQ_INCLUDE_TRYBOTS=skia.primary:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD

Change-Id: Ia4ec1d273b6f137151f951d37c0ebf975f6b9a3e
Reviewed-on: https://skia-review.googlesource.com/8848
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Matt Sarett <msarett@google.com>
9 files changed