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diff --git a/chromium/extensions/common/value_counter.h b/chromium/extensions/common/value_counter.h
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+++ b/chromium/extensions/common/value_counter.h
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+// Copyright 2014 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 EXTENSIONS_COMMON_VALUE_COUNTER_H_
+#define EXTENSIONS_COMMON_VALUE_COUNTER_H_
+
+#include <vector>
+
+#include "base/macros.h"
+#include "base/memory/scoped_ptr.h"
+
+namespace base {
+class Value;
+}
+
+namespace extensions {
+
+// Keeps a running count of Values, like map<Value, int>. Adding/removing
+// values increments/decrements the count associated with a given Value.
+//
+// Add() and Remove() are linear in the number of Values in the ValueCounter,
+// because there is no operator<() defined on Value, so we must iterate to find
+// whether a Value is equal to an existing one.
+class ValueCounter {
+ public:
+  ValueCounter();
+  ~ValueCounter();
+
+  // Adds |value| to the set. In the case where a Value equal to |value|
+  // doesn't already exist in this map, this function makes a copy of |value|
+  // and returns true. Otherwise, it returns false.
+  bool Add(const base::Value& value);
+
+  // Removes |value| from the set, and returns true if it removed the last
+  // value equal to |value|. If there are more equal values, or if there
+  // weren't any in the first place, returns false.
+  bool Remove(const base::Value& value);
+
+  // Returns true if there are no values of any type being counted.
+  bool is_empty() const { return entries_.empty(); }
+
+ private:
+  struct Entry;
+  std::vector<scoped_ptr<Entry>> entries_;
+
+  DISALLOW_COPY_AND_ASSIGN(ValueCounter);
+};
+
+}  // namespace extensions
+
+#endif  // EXTENSIONS_COMMON_VALUE_COUNTER_H_