Support decoding opaque to *premul

src/codec/SkPngCodec.cpp

Index: src/codec/SkPngCodec.cpp
diff --git a/src/codec/SkPngCodec.cpp b/src/codec/SkPngCodec.cpp
index 733efa3e786e71a658f42e0d9d3804e9d9ff1102..710630d47250f05efbcf5b5fa9733a1599fd88ec 100644
--- a/src/codec/SkPngCodec.cpp
+++ b/src/codec/SkPngCodec.cpp
@@ -535,12 +535,12 @@ SkCodec::Result SkPngCodec::onGetPixels(const SkImageInfo& requestedInfo, void*
     return kSuccess;
 }
 
-uint32_t SkPngCodec::onGetFillValue(SkColorType colorType, SkAlphaType alphaType) const {
+uint32_t SkPngCodec::onGetFillValue(SkColorType colorType) const {
     const SkPMColor* colorPtr = get_color_ptr(fColorTable.get());
     if (colorPtr) {
         return get_color_table_fill_value(colorType, colorPtr, 0);
     }
-    return INHERITED::onGetFillValue(colorType, alphaType);
+    return INHERITED::onGetFillValue(colorType);
 }
 
 // Subclass of SkPngCodec which supports scanline decoding