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..bcd303e0a8a409eccd390229336ddb8a4e563b74 100644
--- a/base/chromeos/memory_pressure_observer_chromeos.h
+++ b/base/chromeos/memory_pressure_observer_chromeos.h
@@ -95,6 +95,11 @@ 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 state.
+  base::TimeTicks non_critical_start_time_;
+  base::TimeTicks critical_start_time_;
+
   base::WeakPtrFactory<MemoryPressureObserverChromeOS> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(MemoryPressureObserverChromeOS);