Add some debugging to PremulAlphaRoundTrip test.

tests/PremulAlphaRoundTripTest.cpp

Index: tests/PremulAlphaRoundTripTest.cpp
diff --git a/tests/PremulAlphaRoundTripTest.cpp b/tests/PremulAlphaRoundTripTest.cpp
index 492a4aa3952a86fbecb726a246994069b445085c..1e655fb7382583ac00516c42396e61c02a1fb733 100644
--- a/tests/PremulAlphaRoundTripTest.cpp
+++ b/tests/PremulAlphaRoundTripTest.cpp
@@ -121,6 +121,10 @@ DEF_GPUTEST(PremulAlphaRoundTrip, reporter, factory) {
                     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]);
                     }
                 }