Add Mac memory pressure statistic reporting and consolidate platform code

base/memory/memory_pressure_monitor.cc

Index: base/memory/memory_pressure_monitor.cc
diff --git a/base/memory/memory_pressure_monitor.cc b/base/memory/memory_pressure_monitor.cc
index 00633f1dd61981af41c7738d9c182d97367b9aef..e9ff406585e38922b989ab18dd5358cd99818594 100644
--- a/base/memory/memory_pressure_monitor.cc
+++ b/base/memory/memory_pressure_monitor.cc
@@ -5,12 +5,39 @@
 #include "base/memory/memory_pressure_monitor.h"
 
 #include "base/logging.h"
+#include "base/metrics/histogram_macros.h"
 
 namespace base {
 namespace {
 
 MemoryPressureMonitor* g_monitor = nullptr;
 
+// Enumeration of UMA memory pressure levels. This needs to be kept in sync with
+// histograms.xml and the memory pressure levels defined in
+// MemoryPressureListener.
+enum MemoryPressureLevelUMA {
+  UMA_MEMORY_PRESSURE_LEVEL_NONE = 0,
+  UMA_MEMORY_PRESSURE_LEVEL_MODERATE = 1,
+  UMA_MEMORY_PRESSURE_LEVEL_CRITICAL = 2,
+  // This must be the last value in the enum.
+  UMA_MEMORY_PRESSURE_LEVEL_COUNT,
+};
+
+// Converts a memory pressure level to an UMA enumeration value.
+MemoryPressureLevelUMA MemoryPressureLevelToUmaEnumValue(
+    base::MemoryPressureListener::MemoryPressureLevel level) {
+  switch (level) {
+    case MemoryPressureListener::MEMORY_PRESSURE_LEVEL_NONE:
+      return UMA_MEMORY_PRESSURE_LEVEL_NONE;
+    case MemoryPressureListener::MEMORY_PRESSURE_LEVEL_MODERATE:
+      return UMA_MEMORY_PRESSURE_LEVEL_MODERATE;
+    case MemoryPressureListener::MEMORY_PRESSURE_LEVEL_CRITICAL:
+      return UMA_MEMORY_PRESSURE_LEVEL_CRITICAL;
+  }
+  NOTREACHED();
+  return UMA_MEMORY_PRESSURE_LEVEL_NONE;
+}
+
 }  // namespace
 
 MemoryPressureMonitor::MemoryPressureMonitor() {
@@ -27,5 +54,11 @@ MemoryPressureMonitor::~MemoryPressureMonitor() {
 MemoryPressureMonitor* MemoryPressureMonitor::Get() {
   return g_monitor;
 }
+void MemoryPressureMonitor::RecordMemoryPressure(
+    base::MemoryPressureListener::MemoryPressureLevel level) {
+  UMA_HISTOGRAM_ENUMERATION("Memory.PressureLevel",
+                            MemoryPressureLevelToUmaEnumValue(level),
+                            UMA_MEMORY_PRESSURE_LEVEL_COUNT);
+}
 
 }  // namespace base