[Mac] Report statistics more regularly in Mac memory pressure monitor

base/memory/memory_pressure_monitor.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_H_
 #define BASE_MEMORY_MEMORY_PRESSURE_MONITOR_H_
 
 #include "base/base_export.h"
 #include "base/callback.h"
 #include "base/macros.h"
@@ skipping 17 matching lines @@
 
   virtual ~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;
+  virtual MemoryPressureLevel GetCurrentPressureLevel() = 0;
 
   // Sets a notification callback. The default callback invokes
   // base::MemoryPressureListener::NotifyMemoryPressure.
   virtual void SetDispatchCallback(const DispatchCallback& callback) = 0;
 
  protected:
   MemoryPressureMonitor();
 
  private:
   DISALLOW_COPY_AND_ASSIGN(MemoryPressureMonitor);
 };
 
 }  // namespace base
 
 #endif  // BASE_MEMORY_MEMORY_PRESSURE_MONITOR_H_