Revert of Revert of Fixing libpng transform use

Revert of Revert of Fixing libpng transform use  (patchset #1 id:1 of https://codereview.chromium.org/1213743004/)

Reason for revert:
This appears to have been reverted in order to fix the DEPS roll in https://codereview.chromium.org/1214943004/

However, it only affects SkCodec, which is not used by Chromium. Relanding

Original issue's description:
> Revert of Fixing libpng transform use  (patchset #5 id:80001 of https://codereview.chromium.org/1214203005/)
> 
> Reason for revert:
> DEPS roll failing
> 
> Original issue's description:
> > This change:
> > - supports kGray correctly
> > - avoid extra call to png_get_IHDR by storing the bit depth
> > - call transforms as needed
> > - checks for tRNS alpha value in RGB and GRAY color types
> > 
> > 
> > BUG=skia:
> > 
> > Committed: https://skia.googlesource.com/skia/+/9693037fd41b7ce545b44beaa3489dcfd915018c
> 
> TBR=scroggo@google.com,msarett@google.com,emmaleer@google.com
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=skia:
> 
> Committed: https://skia.googlesource.com/skia/+/6c90e09575c1a77aee060aa475fdb3d25a17d6a0

TBR=msarett@google.com,emmaleer@google.com,jvanverth@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Committed: https://skia.googlesource.com/skia/+/6f29a3c92c976017608a626d0449dda8b603277a

[scroggo, 2015-07-07 13:08:53]

Created Revert of Revert of Fixing libpng transform use

[commit-bot: I haz the power, 2015-07-07 13:09:04]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1215853005/1

[commit-bot: I haz the power, 2015-07-07 13:09:12]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/6f29a3c92c976017608a626d0449dda8b603277a