Fix leak of SkImageGenerator

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1638463002

Review URL: https://codereview.chromium.org/1638463002
diff --git a/dm/DMSrcSink.cpp b/dm/DMSrcSink.cpp
index 057b6ae..5b20e39 100644
--- a/dm/DMSrcSink.cpp
+++ b/dm/DMSrcSink.cpp
@@ -279,7 +279,7 @@
 }
 
 Error test_gen(SkCanvas* canvas, SkData* data) {
-    SkImageGenerator* gen = SkCodecImageGenerator::NewFromEncodedCodec(data);
+    SkAutoTDelete<SkImageGenerator> gen = SkCodecImageGenerator::NewFromEncodedCodec(data);
     if (!gen) {
         return "Could not create image generator.";
     }
@@ -293,7 +293,7 @@
         return Error::Nonfatal("Skip testing non-opaque images to 565.");
     }
 
-    SkAutoTDelete<SkImage> image(SkImage::NewFromGenerator(gen, nullptr));
+    SkAutoTDelete<SkImage> image(SkImage::NewFromGenerator(gen.detach(), nullptr));
     if (!image) {
         return "Could not create image from codec image generator.";
     }