Support decoding opaque to *premul

src/codec/SkPngCodec.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkCodec.h"
 #include "SkColorTable.h"
 #include "SkPngChunkReader.h"
@@ skipping 13 matching lines @@
     // Assume IsPng was called and returned true.
     static SkCodec* NewFromStream(SkStream*, SkPngChunkReader* = NULL);
 
     virtual ~SkPngCodec();
 
 protected:
     Result onGetPixels(const SkImageInfo&, void*, size_t, const Options&, SkPMColor*, int*, int*)
             override;
     SkEncodedFormat onGetEncodedFormat() const override { return kPNG_SkEncodedFormat; }
     bool onRewind() override;
-    uint32_t onGetFillValue(SkColorType colorType, SkAlphaType alphaType) const override;
+    uint32_t onGetFillValue(SkColorType) const override;
 
     // Helper to set up swizzler and color table. Also calls png_read_update_info.
     Result initializeSwizzler(const SkImageInfo& requestedInfo, const Options&,
                               SkPMColor*, int* ctableCount);
     SkSampler* getSampler(bool createIfNecessary) override {
         SkASSERT(fSwizzler);
         return fSwizzler;
     }
 
     SkPngCodec(const SkImageInfo&, SkStream*, SkPngChunkReader*, png_structp, png_infop, int, int);
@@ skipping 14 matching lines @@
 
     SkSwizzler::SrcConfig           fSrcConfig;
     const int                       fNumberPasses;
     int                             fBitDepth;
 
     bool decodePalette(bool premultiply, int* ctableCount);
     void destroyReadStruct();
 
     typedef SkCodec INHERITED;
 };