Add Mac memory pressure statistic reporting and consolidate platform code

base/memory/memory_pressure_monitor_mac.h

Index: base/memory/memory_pressure_monitor_mac.h
diff --git a/base/memory/memory_pressure_monitor_mac.h b/base/memory/memory_pressure_monitor_mac.h
index 89794f67bdd183a948d1ac7b45091b8dc36de07e..9f89b1661a62bfed7898a0f4d825e7f5372b8d40 100644
--- a/base/memory/memory_pressure_monitor_mac.h
+++ b/base/memory/memory_pressure_monitor_mac.h
@@ -5,6 +5,7 @@
 #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"
@@ -35,14 +36,22 @@ class BASE_EXPORT MemoryPressureMonitor : public base::MemoryPressureMonitor {
 
   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);
 };