Add toString() for paint property node classes

cc/output/filter_operations.h

 // 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.
 
 #ifndef CC_OUTPUT_FILTER_OPERATIONS_H_
 #define CC_OUTPUT_FILTER_OPERATIONS_H_
 
 #include <stddef.h>
 
 #include <memory>
@@ skipping 72 matching lines @@
   // linearly interpolating at each position a |progress| fraction of the way
   // from the filter in |from| to the filter in this. If |from| and this are of
   // different lengths, they are treated as having the same length by padding
   // the shorter sequence with no-op filters of the same type as the filters in
   // the corresponding positions in the longer sequence. If either sequence has
   // a reference filter or if there is a type mismatch at some position, returns
   // a copy of this.
   FilterOperations Blend(const FilterOperations& from, double progress) const;
 
   void AsValueInto(base::trace_event::TracedValue* value) const;
+  std::string ToString() const;
 
  private:
   std::vector<FilterOperation> operations_;
 };
 
 }  // namespace cc
 
 #endif  // CC_OUTPUT_FILTER_OPERATIONS_H_