Adding Google benchmarking library.

third_party/google_benchmark/BUILD.gn

+# Copyright 2017 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.
+
+config("google_benchmark_private_config") {
+  include_dirs = [
+    "include",
+    "src",
+  ]
+}
+
+source_set("google_benchmark") {
+  output_name = "google_benchmark"
+  sources = [
+    "include/benchmark/benchmark.h",
+    "include/benchmark/benchmark_api.h",
+    "include/benchmark/macros.h",
+    "include/benchmark/reporter.h",
+    "src/arraysize.h",
+    "src/benchmark.cc",
+    "src/benchmark_api_internal.h",
+    "src/benchmark_register.cc",
+    "src/check.h",
+    "src/colorprint.cc",
+    "src/colorprint.h",
+    "src/commandlineflags.cc",
+    "src/commandlineflags.h",
+    "src/complexity.cc",
+    "src/complexity.h",
+    "src/console_reporter.cc",
+    "src/counter.cc",
+    "src/counter.h",
+    "src/csv_reporter.cc",
+    "src/cycleclock.h",
+    "src/internal_macros.h",
+    "src/json_reporter.cc",
+    "src/log.h",
+    "src/mutex.h",
+    "src/re.h",
+    "src/reporter.cc",
+    "src/sleep.cc",
+    "src/sleep.h",
+    "src/stat.h",
+    "src/string_util.cc",
+    "src/string_util.h",
+    "src/sysinfo.cc",
+    "src/sysinfo.h",
+    "src/timers.cc",
+    "src/timers.h",
+  ]
+
+  configs += [ ":google_benchmark_private_config" ]
+
+  defines = [ "HAVE_STD_REGEX" ]
+
+}