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..9af07d41d4789fd40a886123e4fdb3ae89cf39f5 100644
--- a/base/chromeos/memory_pressure_observer_chromeos.h
+++ b/base/chromeos/memory_pressure_observer_chromeos.h
@@ -76,6 +76,10 @@ class BASE_EXPORT MemoryPressureObserverChromeOS {
   // critical pressure levels.
   void CheckMemoryPressure();
 
+  // The function periodically checks the memory pressure changes and records
+  // the UMA histogram statistics for the current memory pressure level.
+  void CheckMemoryPressureAndRecordStatistics();
+
   // Get the memory pressure in percent (virtual for testing).
   virtual int GetUsedMemoryInPercent();