Test scanline decoding in DM.

BUG=skia:3475

Review URL: https://codereview.chromium.org/999173010
diff --git a/dm/DMSrcSink.h b/dm/DMSrcSink.h
index 1c56be7..3a0eab0 100644
--- a/dm/DMSrcSink.h
+++ b/dm/DMSrcSink.h
@@ -90,13 +90,18 @@
 
 class CodecSrc : public Src {
 public:
-    explicit CodecSrc(Path path);
+    enum Mode {
+        kNormal_Mode,
+        kScanline_Mode,
+    };
+    CodecSrc(Path, Mode);
 
     Error draw(SkCanvas*) const SK_OVERRIDE;
     SkISize size() const SK_OVERRIDE;
     Name name() const SK_OVERRIDE;
 private:
     Path fPath;
+    Mode fMode;
 };