hide Config in SkImageDecoder -- use SkColorType instead

patch from issue 334613003

TBR=scroggo

Author: reed@chromium.org

Review URL: https://codereview.chromium.org/334793002
diff --git a/gm/downsamplebitmap.cpp b/gm/downsamplebitmap.cpp
index 00b1a4b..d7b9307 100644
--- a/gm/downsamplebitmap.cpp
+++ b/gm/downsamplebitmap.cpp
@@ -183,8 +183,7 @@
           }
           if (codec) {
               stream.rewind();
-              codec->decode(&stream, &fBM, SkBitmap::kARGB_8888_Config,
-                  SkImageDecoder::kDecodePixels_Mode);
+              codec->decode(&stream, &fBM, kN32_SkColorType, SkImageDecoder::kDecodePixels_Mode);
               SkDELETE(codec);
           } else {
               fBM.allocN32Pixels(1, 1);