Add Mac memory pressure statistic reporting and consolidate platform code

base/memory/memory_pressure_monitor_chromeos.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/memory/memory_pressure_monitor_chromeos.h"
 
 #include <fcntl.h>
 #include <sys/select.h>
 
 #include "base/metrics/histogram_macros.h"
@@ skipping 141 matching lines @@
 }
 
 void MemoryPressureMonitor::StopObserving() {
   // If StartObserving failed, StopObserving will still get called.
   timer_.Stop();
 }
 
 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:
       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;
@@ skipping 100 matching lines @@
   return percentage;
 }
 
 void MemoryPressureMonitor::SetDispatchCallback(
     const DispatchCallback& callback) {
   dispatch_callback_ = callback;
 }
 
 }  // namespace chromeos
 }  // namespace base