send SkPMColor::toPMColor() through Sk4f_toL32()

This doesn't fix that 8888 gradient if we stop clamping,
but it seems like a good idea to land first anyway.

Change-Id: Ie0feda67da5996223db2fe4458f99d57cf13db71
Reviewed-on: https://skia-review.googlesource.com/155782
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
4 files changed