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 66215a6c73eef2ffa6206ab30795b6b7d544932c..510cd030b92d2b1cd37234c0f25acc595f010a20 100644
--- a/src/codec/SkPngCodec.cpp
+++ b/src/codec/SkPngCodec.cpp
@@ -565,12 +565,12 @@ 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