Adding Google benchmarking library.

third_party/google_benchmark/src/string_util.h

+#ifndef BENCHMARK_STRING_UTIL_H_
+#define BENCHMARK_STRING_UTIL_H_
+
+#include <sstream>
+#include <string>
+#include <utility>
+#include "internal_macros.h"
+
+namespace benchmark {
+
+void AppendHumanReadable(int n, std::string* str);
+
+std::string HumanReadableNumber(double n);
+
+std::string StringPrintF(const char* format, ...);
+
+inline std::ostream& StringCatImp(std::ostream& out) BENCHMARK_NOEXCEPT {
+  return out;
+}
+
+template <class First, class... Rest>
+inline std::ostream& StringCatImp(std::ostream& out, First&& f,
+                                  Rest&&... rest) {
+  out << std::forward<First>(f);
+  return StringCatImp(out, std::forward<Rest>(rest)...);
+}
+
+template <class... Args>
+inline std::string StrCat(Args&&... args) {
+  std::ostringstream ss;
+  StringCatImp(ss, std::forward<Args>(args)...);
+  return ss.str();
+}
+
+void ReplaceAll(std::string* str, const std::string& from,
+                const std::string& to);
+
+}  // end namespace benchmark
+
+#endif  // BENCHMARK_STRING_UTIL_H_