Adding png scanline decoding to kIndex8

dm/DM.cpp

Index: dm/DM.cpp
diff --git a/dm/DM.cpp b/dm/DM.cpp
index c0fccaaa2c7bfcee54624fb32f02af5f4771161a..0ceb815b1b6263cebc848a5f3a99c4dda6d49863 100644
--- a/dm/DM.cpp
+++ b/dm/DM.cpp
@@ -203,9 +203,8 @@ static void push_codec_srcs(Path path) {
       case kIndex_8_SkColorType:
           push_src("image", "codec_kIndex8", new CodecSrc(path, CodecSrc::kNormal_Mode,
                   CodecSrc::kIndex8_Always_DstColorType));
-          // FIXME: Need to implement scanline decoding for kIndex8.
-          //push_src("image", "scanline kIndex8", new CodecSrc(path, CodecSrc::kScanline_Mode,
-          //        CodecSrc::kIndex8_Always_DstColorType));
+          push_src("image", "scanline_kIndex8", new CodecSrc(path, CodecSrc::kScanline_Mode,
+                  CodecSrc::kIndex8_Always_DstColorType));
         break;
       default:
         // Do nothing