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

base/chromeos/memory_pressure_observer_chromeos.h

Index: base/chromeos/memory_pressure_observer_chromeos.h
diff --git a/base/chromeos/memory_pressure_observer_chromeos.h b/base/chromeos/memory_pressure_observer_chromeos.h
index 739b795067e4e7202716760c5882b205a014a55b..126ab2a42459968960af3831826fd4d46dd20d99 100644
--- a/base/chromeos/memory_pressure_observer_chromeos.h
+++ b/base/chromeos/memory_pressure_observer_chromeos.h
@@ -95,6 +95,16 @@ class BASE_EXPORT MemoryPressureObserverChromeOS {
   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);