Add UMA stats to get the quality of service of memory pressure handling scheme in ChromeOS.

base/chromeos/memory_pressure_observer_chromeos.h

 // Copyright 2014 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.
 
 #ifndef BASE_CHROMEOS_MEMORY_PRESSURE_OBSERVER_CHROMEOS_H_
 #define BASE_CHROMEOS_MEMORY_PRESSURE_OBSERVER_CHROMEOS_H_
 
 #include "base/base_export.h"
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
@@ skipping 77 matching lines @@
   base::RepeatingTimer<MemoryPressureObserverChromeOS> timer_;
 
   // To slow down the amount of moderate pressure event calls, this counter
   // gets used to count the number of events since the last event occured.
   int moderate_pressure_repeat_count_;
 
   // The thresholds for moderate and critical pressure.
   const int moderate_pressure_threshold_percent_;
   const int critical_pressure_threshold_percent_;
 
+  // Wall-clock time when the system entered non-critical or critical memory
+  // pressure mode.
+  base::TimeTicks non_critical_start_time_;
+  base::TimeTicks critical_start_time_;
+
+  // The length of time that the system stayed in non-critical or critical
+  // memory pressure mode.
+  base::TimeDelta non_critical_delta_;

[Mr4D (OOO till 08-26), 2015/01/29 08:17:53]

We do not need this value. It can be derived from the existing
non_critical_start_time and critical_start_time at the time when we write the
UMA stats.

[xdai1, 2015/01/31 00:01:39]

You're right. Deleted these two values.

+  base::TimeDelta critical_delta_;

[Mr4D (OOO till 08-26), 2015/01/29 08:17:53]

On a second thought - we do not need to store either of these.

[xdai1, 2015/01/31 00:01:39]

Done.

+
   base::WeakPtrFactory<MemoryPressureObserverChromeOS> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(MemoryPressureObserverChromeOS);
 };
 
 }  // namespace base
 
 #endif  // BASE_CHROMEOS_MEMORY_PRESSURE_OBSERVER_CHROMEOS_H_