Add a Via to DM that records into two pictures and draws using the second.

dm/DMSrcSink.h

Index: dm/DMSrcSink.h
diff --git a/dm/DMSrcSink.h b/dm/DMSrcSink.h
index 2c21e87ef60c017926fa4dc841667d2f1d94588e..57af9110627dbdf3887e0e1aa28a9edcc9c706b4 100644
--- a/dm/DMSrcSink.h
+++ b/dm/DMSrcSink.h
@@ -266,6 +266,17 @@ private:
     SkAutoTDelete<Sink>         fSink;
 };
 
+class ViaSecondPicture : public Sink {
+public:
+    explicit ViaSecondPicture(Sink*);
+
+    Error draw(const Src&, SkBitmap*, SkWStream*, SkString*) const override;
+    int enclave() const override { return fSink->enclave(); }
+    const char* fileExtension() const override { return fSink->fileExtension(); }
+private:
+    SkAutoTDelete<Sink>  fSink;
+};
+
 }  // namespace DM
 
 #endif//DMSrcSink_DEFINED