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/DM.cpp b/dm/DM.cpp
index 70057dd..07518c9 100644
--- a/dm/DM.cpp
+++ b/dm/DM.cpp
@@ -370,6 +370,7 @@
     VIA("serialize", ViaSerialization, wrapped);
     VIA("deferred",  ViaDeferred,      wrapped);
     VIA("2ndpic",    ViaSecondPicture, wrapped);
+    VIA("sp",        ViaSingletonPictures, wrapped);
     VIA("tiles",     ViaTiles, 256, 256,               NULL, wrapped);
     VIA("tiles_rt",  ViaTiles, 256, 256, new SkRTreeFactory, wrapped);