try Herb's new to_srgb

try Herb's new to_srgb

This was 6-8% faster than the previous code on my Trashcan.

Change-Id: I70081009e233c83226d6d302f871fb7e86cdc438
Reviewed-on: https://skia-review.googlesource.com/16986
Reviewed-by: Matt Sarett <msarett@google.com>
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>