Add toString() for paint property node classes

cc/output/filter_operations_unittest.cc

 // Copyright 2013 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 <stddef.h>
 
 #include "cc/output/filter_operations.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "third_party/skia/include/effects/SkBlurImageFilter.h"
 #include "third_party/skia/include/effects/SkDropShadowImageFilter.h"
@@ skipping 938 matching lines @@
   from.Append(FilterOperation::CreateOpacityFilter(1.f));
   to.Append(FilterOperation::CreateOpacityFilter(1.f));
   blended = to.Blend(from, -0.75);
   EXPECT_EQ(to, blended);
   blended = to.Blend(from, 0.75);
   EXPECT_EQ(to, blended);
   blended = to.Blend(from, 1.5);
   EXPECT_EQ(to, blended);
 }
 
+TEST(FilterOperationsTest, ToString) {
+  FilterOperations filters;
+  EXPECT_EQ(std::string("{\"FilterOperations\":[]}"), filters.ToString());
+
+  filters.Append(FilterOperation::CreateSaturateFilter(3.f));
+  filters.Append(FilterOperation::CreateBlurFilter(2.f));
+  EXPECT_EQ(std::string("{\"FilterOperations\":[{\"amount\":3.0,\"type\":2},"
+                        "{\"amount\":2.0,\"type\":8}]}"),
+            filters.ToString());
+}
+
 }  // namespace
 }  // namespace cc