DM: add a Via to simulate SP pictures.

dm/DMSrcSink.h

Index: dm/DMSrcSink.h
diff --git a/dm/DMSrcSink.h b/dm/DMSrcSink.h
index ea5572a7b906ff964c569d04b8afe4312cfc76db..0d612d0b933997358086d8ad35c4f0dfc861959e 100644
--- a/dm/DMSrcSink.h
+++ b/dm/DMSrcSink.h
@@ -295,6 +295,17 @@ private:
     SkAutoTDelete<Sink>  fSink;
 };
 
+class ViaSingletonPictures : public Sink {
+public:
+    explicit ViaSingletonPictures(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