Adding Google benchmarking library.

third_party/google_benchmark/include/benchmark/benchmark.h

+// Copyright 2015 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+#ifndef BENCHMARK_BENCHMARK_H_
+#define BENCHMARK_BENCHMARK_H_
+
+#include "benchmark_api.h"
+#include "macros.h"
+#include "reporter.h"
+
+#endif  // BENCHMARK_BENCHMARK_H_