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:

Review URL: https://codereview.chromium.org/1215853005

1 files changed, 5 insertions, 2 deletions

diff --git a/src/codec/SkCodec_libpng.h b/src/codec/SkCodec_libpng.h
index a105c3c5e3..c0fee74e96 100644
--- a/src/codec/SkCodec_libpng.h
+++ b/src/codec/SkCodec_libpng.h
@@ -44,14 +44,17 @@ private:
     SkSwizzler::SrcConfig       fSrcConfig;
     int                         fNumberPasses;
     bool                        fReallyHasAlpha;
+    int                         fBitDepth;
 
-    SkPngCodec(const SkImageInfo&, SkStream*, png_structp, png_infop);
+    SkPngCodec(const SkImageInfo&, SkStream*, png_structp, png_infop, int);
     ~SkPngCodec();
 
+
     // Helper to set up swizzler and color table. Also calls png_read_update_info.
     Result initializeSwizzler(const SkImageInfo& requestedInfo, void* dst,
                               size_t rowBytes, const Options&, SkPMColor*, int* ctableCount);
-    // Calls rewindIfNeeded, and returns true if the decoder can continue.
+
+    // Calls rewindIfNeeded and returns true if the decoder can continue.
     bool handleRewind();
     bool decodePalette(bool premultiply, int bitDepth, int* ctableCount);
     void finish();