Use new jpeg_crop_scanlines() API to optimize jpeg subset decodes

This was adapted from:
https://codereview.chromium.org/1530933003

Subset Decode Runtime (Original / Optimized) on Nexus 6P
TopLeft     0.51x
TopRight    0.56x
Middle      0.71x
BottomLeft  0.79x
BottomRight 0.79x

BUG=skia:4256
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1719073002

Review URL: https://codereview.chromium.org/1719073002
diff --git a/dm/DMSrcSink.h b/dm/DMSrcSink.h
index a987c7d..3190576 100644
--- a/dm/DMSrcSink.h
+++ b/dm/DMSrcSink.h
@@ -110,6 +110,7 @@
         kCodecZeroInit_Mode,
         kScanline_Mode,
         kStripe_Mode, // Tests the skipping of scanlines
+        kCroppedScanline_Mode, // Tests (jpeg) cropped scanline optimization
         kSubset_Mode, // For codecs that support subsets directly.
         kGen_Mode,    // Test SkCodecImageGenerator (includes YUV)
     };