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

webrtc/base/cpu_time.h

Index: webrtc/base/cpu_time.h
diff --git a/webrtc/base/cpu_time.h b/webrtc/base/cpu_time.h
new file mode 100644
index 0000000000000000000000000000000000000000..9d06223098b68c51fe494283d6af7558854e5242
--- /dev/null
+++ b/webrtc/base/cpu_time.h
@@ -0,0 +1,28 @@
+/*
+ *  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.
+// Use to calculate difference as some offset may be presented.

[Taylor Brandstetter, 2017/02/16 18:49:19]

I don't quite understand the "use to calculate difference" comment. Is this
saying that there are no guarantees about the initial time offset, and so this
time should only be used for calculating differences? If so, it may be useful to
make the return value a class that only *allows* taking differences.

[ilnik, 2017/02/17 09:17:10]

I guess, just mentioning that may not start from 0 will be enough here.

Just like with most of the other clock functions, there is some offset. For CPU
time consumed by process it may be logical to start from 0 at the start of the
process, but it's not like that for all the platforms. I think making special
return type will be overkill here if that functions will be used in the same way
other time functions are used.

+int64_t GetProcessCpuTime();

[Taylor Brandstetter, 2017/02/16 18:49:19]

Can we call this GetProcessCpuTimeNanos, so the units are clear from the call
site? We use this kind of naming for "SystemTimeNanos" and "TimeNanos".

[ilnik, 2017/02/17 09:17:10]

Done.

+
+//  Returns total CPU time of a current thread in nanoseconds.

[Taylor Brandstetter, 2017/02/16 18:49:19]

nit: extra spaces in comments.

[ilnik, 2017/02/17 09:17:10]

Done.

+//  Use to calculate difference as some offset may be presented.
+int64_t GetThreadCpuTime();
+
+}  // namespace rtc
+
+#endif  // WEBRTC_BASE_CPU_TIME_H_