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

I'm going to start hacking on SkCanvas a bit to allow a fast reset method,
and I want to have some testing checking me.

BUG=skia:

Review URL: https://codereview.chromium.org/1062043004
diff --git a/dm/DMSrcSink.h b/dm/DMSrcSink.h
index 2c21e87..57af911 100644
--- a/dm/DMSrcSink.h
+++ b/dm/DMSrcSink.h
@@ -266,6 +266,17 @@
     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