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/tools/skdiff_utils.cpp b/tools/skdiff_utils.cpp
index 5c9aae2..9157ac6 100644
--- a/tools/skdiff_utils.cpp
+++ b/tools/skdiff_utils.cpp
@@ -63,7 +63,7 @@
     SkAutoTDelete<SkImageDecoder> ad(codec);
 
     stream.rewind();
-    if (!codec->decode(&stream, &resource.fBitmap, SkBitmap::kARGB_8888_Config, mode)) {
+    if (!codec->decode(&stream, &resource.fBitmap, kN32_SkColorType, mode)) {
         SkDebugf("ERROR: codec failed for basePath <%s>\n", resource.fFullPath.c_str());
         resource.fStatus = DiffResource::kCouldNotDecode_Status;
         return false;