port ok to SkPngEncoder
port ok to SkPngEncoder

I can't tell you how excited I am to turn down the libpng settings...
anything that saves me a few seconds is a nice quality of life win.

This CL makes ok run in about half the time when producing .pngs.

Profile running `ok gm srgb png` before:
    10.59   s   16.5%    10.59   s          longest_match
    8.98    s   14.0%    8.98    s          png_setup_paeth_row
    8.93    s   13.9%    8.93    s          skia_png_write_find_filter
    7.75    s   12.1%    7.75    s          deflate_slow
    4.63    s    7.2%    4.63    s          std::wait_until(...) const
    959.00 ms    1.5%    959.00 ms          SkPathRef::validate() const
    935.00 ms    1.4%    935.00 ms          sk_to_srgb_hsw
    ...

After:
    2.35    s    8.7%    2.35    s          std::wait_until(...) const
    1.70    s    6.2%    1.70    s          longest_match
    1.19    s    4.4%    1.19    s          deflate_fast
    931.00 ms    3.4%    931.00 ms          SkPathRef::validate() const
    898.00 ms    3.3%    898.00 ms          sk_to_srgb_hsw
    ...

Change-Id: I425c30b2ecd97a0e4a4392779de6301db473ee47
Reviewed-on: https://skia-review.googlesource.com/15547
Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@chromium.org>
Reviewed-by: Matt Sarett <bb43c9b8214fcf1292f134c9714ba8a0e7075913@google.com>

HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,rubenvb/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,google/skia,rubenvb/skia,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,google/skia,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,rubenvb/skia,google/skia,aosp-mirror/platform_external_skia,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,google/skia,google/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,HalCanary/skia-hc,google/skia,google/skia,rubenvb/skia,rubenvb/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,rubenvb/skia,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,google/skia,HalCanary/skia-hc,HalCanary/skia-hc,HalCanary/skia-hc,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia