Add UMA for High Resolution Timer Usage

base/time/time.h

 // Copyright (c) 2012 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.
 
 // Time represents an absolute point in coordinated universal time (UTC),
 // internally represented as microseconds (s/1,000,000) since the Windows epoch
 // (1601-01-01 00:00:00 UTC). System-dependent clock interface routines are
 // defined in time_PLATFORM.cc. Note that values for Time may skew and jump
 // around as the operating system makes adjustments to synchronize (e.g., with
 // NTP servers). Thus, client code that uses the Time class must account for
@@ skipping 531 matching lines @@
   // returns true.  Each successful activate call must be paired with a
   // subsequent deactivate call.
   // All callers to activate the high resolution timer must eventually call
   // this function to deactivate the high resolution timer.
   static bool ActivateHighResolutionTimer(bool activate);
 
   // Returns true if the high resolution timer is both enabled and activated.
   // This is provided for testing only, and is not tracked in a thread-safe
   // way.
   static bool IsHighResolutionTimerInUse();
+
+  // The following two functions are used to report the fraction of elapsed time
+  // that the high resolution timer is activated.
+  // ResetHighResolutionTimerUsage() resets the cumulative usage and starts the
+  // measurement interval and GetHighResolutionTimerUsage() returns the
+  // percentage of time since the reset that the high resolution timer was
+  // activated.

[gab, 2017/06/28 16:39:33]

Also document that Get() is invalid before Reset() was called at least once?
(although your test verify that case, it's undefined to use Get() before Reset()
when the highres timer was activated already)

[stanisc, 2017/06/28 22:28:35]

Done.

+  static void ResetHighResolutionTimerUsage();
+  static double GetHighResolutionTimerUsage();
 #endif

[gab, 2017/06/28 16:39:33]

#endif  // defined(OS_WIN)

(this block is becoming larger and larger and a closing comment is now
justified)

[stanisc, 2017/06/28 22:28:35]

Done.

 
   // Converts an exploded structure representing either the local time or UTC
   // into a Time class. Returns false on a failure when, for example, a day of
   // month is set to 31 on a 28-30 day month. Returns Time(0) on overflow.
   static bool FromUTCExploded(const Exploded& exploded,
                               Time* time) WARN_UNUSED_RESULT {
     return FromExploded(false, exploded, time);
   }
   static bool FromLocalExploded(const Exploded& exploded,
                                 Time* time) WARN_UNUSED_RESULT {
@@ skipping 308 matching lines @@
   static void WaitUntilInitializedWin();
 #endif
 };
 
 // For logging use only.
 BASE_EXPORT std::ostream& operator<<(std::ostream& os, ThreadTicks time_ticks);
 
 }  // namespace base
 
 #endif  // BASE_TIME_TIME_H_