cc: Log pngs in pixel test only if they are different.

cc/test/pixel_test_utils.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/test/pixel_test_utils.h"
 
 #include <string>
 #include <vector>
 
 #include "base/base64.h"
@@ skipping 54 matching lines @@
         << "Actual: " << gen_bmp.width() << "x" << gen_bmp.height()
         << "; "
         << "Expected: " << ref_bmp.width() << "x" << ref_bmp.height();
     return false;
   }
 
   // Shortcut for empty images. They are always equal.
   if (gen_bmp.width() == 0 || gen_bmp.height() == 0)
     return true;
 
-  std::string gen_bmp_data_url = GetPNGDataUrl(gen_bmp);
-  std::string ref_bmp_data_url = GetPNGDataUrl(ref_bmp);
-  LOG(ERROR) << "Pixels do not match!";
-  LOG(ERROR) << "Actual: " << gen_bmp_data_url;
-  LOG(ERROR) << "Expected: " << ref_bmp_data_url;
-
-  return comparator.Compare(gen_bmp, ref_bmp);
+  bool compare = comparator.Compare(gen_bmp, ref_bmp);
+  if (!compare) {
+    std::string gen_bmp_data_url = GetPNGDataUrl(gen_bmp);
+    std::string ref_bmp_data_url = GetPNGDataUrl(ref_bmp);
+    LOG(ERROR) << "Pixels do not match!";
+    LOG(ERROR) << "Actual: " << gen_bmp_data_url;
+    LOG(ERROR) << "Expected: " << ref_bmp_data_url;
+  }
+  return compare;
 }
 
 }  // namespace cc