Do we still need this DOUBLE_LOOP feature?

If so, let's do it this way so it works for all source types and doesn't need
to be chosen at compile time.

BUG=skia:

Review URL: https://codereview.chromium.org/1129693003
diff --git a/dm/DMSrcSink.h b/dm/DMSrcSink.h
index 903755e..0537e26 100644
--- a/dm/DMSrcSink.h
+++ b/dm/DMSrcSink.h
@@ -278,6 +278,12 @@
     Error draw(const Src&, SkBitmap*, SkWStream*, SkString*) const override;
 };
 
+class ViaTwice : public Via {
+public:
+    explicit ViaTwice(Sink* sink) : Via(sink) {}
+    Error draw(const Src&, SkBitmap*, SkWStream*, SkString*) const override;
+};
+
 }  // namespace DM
 
 #endif//DMSrcSink_DEFINED