Add some debugging to PremulAlphaRoundTrip test.

tests/PremulAlphaRoundTripTest.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkBitmapDevice.h"
 #include "SkCanvas.h"
 #include "SkConfig8888.h"
@@ skipping 103 matching lines @@
                 canvas.readPixels(&readBmp1, 0, 0);
                 sk_tool_utils::write_pixels(&canvas, readBmp1, 0, 0, gUnpremul[upmaIdx].fColorType,
                                             kUnpremul_SkAlphaType);
                 canvas.readPixels(&readBmp2, 0, 0);
 
                 bool success = true;
                 for (int y = 0; y < 256 && success; ++y) {
                     const uint32_t* pixels1 = readBmp1.getAddr32(0, y);
                     const uint32_t* pixels2 = readBmp2.getAddr32(0, y);
                     for (int x = 0; x < 256 && success; ++x) {
+                        // We see sporadic failures here. May help to see where it goes wrong.
+                        if (pixels1[x] != pixels2[x]) {
+                            SkDebugf("%x != %x, x = %d, y = %d\n", pixels1[x], pixels2[x], x, y);
+                        }
                         REPORTER_ASSERT(reporter, success = pixels1[x] == pixels2[x]);
                     }
                 }
             }
         }
     }
 }