Add Mac memory pressure statistic reporting and consolidate platform code

base/memory/memory_pressure_monitor_mac.h

 // Copyright 2015 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.
 
 #ifndef BASE_MEMORY_MEMORY_PRESSURE_MONITOR_MAC_H_
 #define BASE_MEMORY_MEMORY_PRESSURE_MONITOR_MAC_H_
 
+#include <CoreFoundation/CFDate.h>
 #include <dispatch/dispatch.h>
 
 #include "base/base_export.h"
 #include "base/mac/scoped_dispatch_object.h"
 #include "base/macros.h"
 #include "base/memory/memory_pressure_listener.h"
 #include "base/memory/memory_pressure_monitor.h"
 
 namespace base {
 namespace mac {
@@ skipping 10 matching lines @@
   // Returns the currently-observed memory pressure.
   MemoryPressureLevel GetCurrentPressureLevel() const override;
 
   void SetDispatchCallback(const DispatchCallback& callback) override;
 
  private:
   friend TestMemoryPressureMonitor;
 
   static MemoryPressureLevel
       MemoryPressureLevelForMacMemoryPressure(int mac_memory_pressure);
-  static void NotifyMemoryPressureChanged(
-      dispatch_source_s* event_source,
-      const DispatchCallback& dispatch_callback);
+  void OnMemoryPressureChanged(dispatch_source_s* event_source,
+                               const DispatchCallback& dispatch_callback);
 
   ScopedDispatchObject<dispatch_source_t> memory_level_event_source_;
 
   DispatchCallback dispatch_callback_;
 
+  CFTimeInterval last_pressure_change_;
+
+  MemoryPressureLevel last_pressure_level_;
+
+  // The UMA statistic is recorded in 5 second increments. This
+  // accumulates the remaining time to be rolled into the next
+  // call.
+  CFTimeInterval reporting_error_;
+
   DISALLOW_COPY_AND_ASSIGN(MemoryPressureMonitor);
 };
 
 }  // namespace mac
 }  // namespace base
 
 #endif  // BASE_MEMORY_MEMORY_PRESSURE_MONITOR_MAC_H_