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..53958168839f45565ad1d01675febfe33a11450d 100644
--- a/base/chromeos/memory_pressure_observer_chromeos.h
+++ b/base/chromeos/memory_pressure_observer_chromeos.h
@@ -95,6 +95,13 @@ class BASE_EXPORT MemoryPressureObserverChromeOS {
   const int moderate_pressure_threshold_percent_;
   const int critical_pressure_threshold_percent_;
 
+  // The time that the system stays in non-critical memory pressure state in a
+  // session (in Millisecond).
+  unsigned int time_non_critical_pressure_;
+  // The time that the system stays in critical memory pressure state in a
+  // session (in Millisecond).
+  unsigned int time_critical_pressure_;
+
   base::WeakPtrFactory<MemoryPressureObserverChromeOS> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(MemoryPressureObserverChromeOS);