DM: add a Via to simulate SP pictures.

Will use this to test the other CL that adds small SkPicture implementations.

Not quite sure why patch_primitive doesn't draw the same in 8888 and sp-8888, but everything else does, so I'm not going to let that hold me back for now.

BUG=skia:

Review URL: https://codereview.chromium.org/1126613005
diff --git a/dm/DMSrcSink.h b/dm/DMSrcSink.h
index ea5572a..0d612d0 100644
--- a/dm/DMSrcSink.h
+++ b/dm/DMSrcSink.h
@@ -295,6 +295,17 @@
     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