Add toString() for paint property node classes

cc/output/filter_operations_unittest.cc

Index: cc/output/filter_operations_unittest.cc
diff --git a/cc/output/filter_operations_unittest.cc b/cc/output/filter_operations_unittest.cc
index 89ed4608d4ffe47c88037d2bb1ad4dfc167e650e..8a32e663d5447e6c6ec09462d3448927b4cfb0b6 100644
--- a/cc/output/filter_operations_unittest.cc
+++ b/cc/output/filter_operations_unittest.cc
@@ -956,5 +956,16 @@ TEST(FilterOperationsTest, BlendRaggedSequences) {
   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