https://codereview.chromium.org/1830973002 fixed this bug. Re-enable the

dm/DMSrcSink.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "DMSrcSink.h"
 #include "SkAndroidCodec.h"
 #include "SkCodec.h"
@@ skipping 813 matching lines @@
             }
             break;
         }
         default:
             SkASSERT(false);
             return "Invalid image generator mode";
     }
 
     // Test deferred decoding path on GPU
     if (fIsGpu) {
-        // FIXME: The gpu backend does not draw kGray sources correctly. (skbug.com/4822)
-        //        We have disabled these tests in DM.cpp.
-        SkASSERT(kGray_8_SkColorType != gen->getInfo().colorType());
-
         sk_sp<SkImage> image(SkImage::MakeFromGenerator(gen.release(), nullptr));
         if (!image) {
             return "Could not create image from codec image generator.";
         }
         canvas->drawImage(image, 0, 0);
         return "";
     }
 
     // Test various color and alpha types on CPU
     SkImageInfo decodeInfo = gen->getInfo().makeAlphaType(fDstAlphaType);
@@ skipping 662 matching lines @@
             skr.visit(i, drawsAsSingletonPictures);
         }
         sk_sp<SkPicture> macroPic(macroRec.finishRecordingAsPicture());
 
         canvas->drawPicture(macroPic);
         return check_against_reference(bitmap, src, fSink);
     });
 }
 
 }  // namespace DM