Add CodecZeroInit test to DMSrcSink

This should not cause any diffs on Gold.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1577853004

Review URL: https://codereview.chromium.org/1577853004
diff --git a/dm/DMSrcSink.h b/dm/DMSrcSink.h
index 6645807..def7cc9 100644
--- a/dm/DMSrcSink.h
+++ b/dm/DMSrcSink.h
@@ -104,6 +104,10 @@
 public:
     enum Mode {
         kCodec_Mode,
+        // We choose to test only one mode with zero initialized memory.
+        // This will exercise all of the interesting cases in SkSwizzler
+        // without doubling the size of our test suite.
+        kCodecZeroInit_Mode,
         kScanline_Mode,
         kStripe_Mode, // Tests the skipping of scanlines
         kSubset_Mode, // For codecs that support subsets directly.