Add Mac memory pressure statistic reporting and consolidate platform code

base/memory/memory_pressure_monitor.h

Index: base/memory/memory_pressure_monitor.h
diff --git a/base/memory/memory_pressure_monitor.h b/base/memory/memory_pressure_monitor.h
index a4adba4f3ad8d79913e1d2030568eead002d18b4..033379cdb8d51e08151d8ab36bf610264eac909e 100644
--- a/base/memory/memory_pressure_monitor.h
+++ b/base/memory/memory_pressure_monitor.h
@@ -31,6 +31,9 @@ class BASE_EXPORT MemoryPressureMonitor {
   // Return the singleton MemoryPressureMonitor.
   static MemoryPressureMonitor* Get();
 
+  // Record memory pressure UMA statistic. A tick is 5 seconds.
+  static void RecordMemoryPressure(MemoryPressureLevel level, int ticks);
+
   // Returns the currently observed memory pressure.
   virtual MemoryPressureLevel GetCurrentPressureLevel() const = 0;