Added GetCpuTime to base/ to get total CPU time consumed by process for perf tests.

webrtc/base/cpu_time.h

+/*
+ *  Copyright (c) 2017 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef WEBRTC_BASE_CPU_TIME_H_
+#define WEBRTC_BASE_CPU_TIME_H_
+
+#include <stdint.h>
+
+namespace rtc {
+
+// Returns total CPU time of a current process in nanoseconds.
+// Time base is unknown, therefore use only to calculate deltas.
+int64_t GetProcessCpuTimeNanos();
+
+// Returns total CPU time of a current thread in nanoseconds.
+// Time base is unknown, therefore use only to calculate deltas.
+int64_t GetThreadCpuTimeNanos();
+
+}  // namespace rtc
+
+#endif  // WEBRTC_BASE_CPU_TIME_H_