Add Mac memory pressure statistic reporting and consolidate platform code

base/memory/memory_pressure_monitor_chromeos.cc

Index: base/memory/memory_pressure_monitor_chromeos.cc
diff --git a/base/memory/memory_pressure_monitor_chromeos.cc b/base/memory/memory_pressure_monitor_chromeos.cc
index 0e1aa815ab0e4882efc7b9fb9f14816747cc1170..04f1919a31795aaf4be86c3c91ce646e387ac50b 100644
--- a/base/memory/memory_pressure_monitor_chromeos.cc
+++ b/base/memory/memory_pressure_monitor_chromeos.cc
@@ -159,7 +159,10 @@ void MemoryPressureMonitor::StopObserving() {
 void MemoryPressureMonitor::CheckMemoryPressureAndRecordStatistics() {
   CheckMemoryPressure();
 
+  RecordMemoryPressure(current_memory_pressure_level_);
   // Record UMA histogram statistics for the current memory pressure level.
+  // TODO(lgrey): Remove this once there's a usable history for the
+  // "Memory.PressureLevel" statistic
   MemoryPressureLevelUMA memory_pressure_level_uma(MEMORY_PRESSURE_LEVEL_NONE);
   switch (current_memory_pressure_level_) {
     case MemoryPressureListener::MEMORY_PRESSURE_LEVEL_NONE: