Update from https://crrev.com/315085

base/chromeos/memory_pressure_observer_chromeos.cc

Index: base/chromeos/memory_pressure_observer_chromeos.cc
diff --git a/base/chromeos/memory_pressure_observer_chromeos.cc b/base/chromeos/memory_pressure_observer_chromeos.cc
index 8112c761e5e713e747cb015d8226e62a754c87a8..5691eb8b4ed1dbd626e85ef1ea617886396eeafd 100644
--- a/base/chromeos/memory_pressure_observer_chromeos.cc
+++ b/base/chromeos/memory_pressure_observer_chromeos.cc
@@ -5,6 +5,7 @@
 #include "base/chromeos/memory_pressure_observer_chromeos.h"
 
 #include "base/message_loop/message_loop.h"
+#include "base/metrics/histogram_macros.h"
 #include "base/process/process_metrics.h"
 #include "base/time/time.h"
 
@@ -29,6 +30,16 @@ const int kNormalMemoryPressureCriticalThresholdPercent = 90;
 const int kAggressiveMemoryPressureModerateThresholdPercent = 35;
 const int kAggressiveMemoryPressureCriticalThresholdPercent = 70;
 
+// The possible state for memory pressure level. The values should be in line
+// with values in MemoryPressureListener::MemoryPressureLevel and should be
+// updated if more memory pressure levels are introduced.
+enum MemoryPressureLevelUMA {
+  MEMORY_PRESSURE_LEVEL_NONE = 0,
+  MEMORY_PRESSURE_LEVEL_MODERATE,
+  MEMORY_PRESSURE_LEVEL_CRITICAL,
+  NUM_MEMORY_PRESSURE_LEVELS
+};
+
 // Converts a |MemoryPressureThreshold| value into a used memory percentage for
 // the moderate pressure event.
 int GetModerateMemoryThresholdInPercent(
@@ -92,7 +103,8 @@ void MemoryPressureObserverChromeOS::ScheduleEarlyCheck() {
 void MemoryPressureObserverChromeOS::StartObserving() {
   timer_.Start(FROM_HERE,
                TimeDelta::FromMilliseconds(kMemoryPressureIntervalMs),
-               Bind(&MemoryPressureObserverChromeOS::CheckMemoryPressure,
+               Bind(&MemoryPressureObserverChromeOS::
+                        CheckMemoryPressureAndRecordStatistics,
                     weak_ptr_factory_.GetWeakPtr()));
 }
 
@@ -101,6 +113,28 @@ void MemoryPressureObserverChromeOS::StopObserving() {
   timer_.Stop();
 }
 
+void MemoryPressureObserverChromeOS::CheckMemoryPressureAndRecordStatistics() {
+  CheckMemoryPressure();
+
+  // Record UMA histogram statistics for the current memory pressure level.
+  MemoryPressureLevelUMA memory_pressure_level_uma(MEMORY_PRESSURE_LEVEL_NONE);
+  switch (current_memory_pressure_level_) {
+    case MemoryPressureListener::MEMORY_PRESSURE_LEVEL_NONE:
+      memory_pressure_level_uma = MEMORY_PRESSURE_LEVEL_NONE;
+      break;
+    case MemoryPressureListener::MEMORY_PRESSURE_LEVEL_MODERATE:
+      memory_pressure_level_uma = MEMORY_PRESSURE_LEVEL_MODERATE;
+      break;
+    case MemoryPressureListener::MEMORY_PRESSURE_LEVEL_CRITICAL:
+      memory_pressure_level_uma = MEMORY_PRESSURE_LEVEL_CRITICAL;
+      break;
+  }
+
+  UMA_HISTOGRAM_ENUMERATION("ChromeOS.MemoryPressureLevel",
+                            memory_pressure_level_uma,
+                            NUM_MEMORY_PRESSURE_LEVELS);
+}
+
 void MemoryPressureObserverChromeOS::CheckMemoryPressure() {
   MemoryPressureListener::MemoryPressureLevel old_pressure =
       current_memory_pressure_level_;