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

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 116 matching lines @@
   StopObserving();
 }
 
 void MemoryPressureMonitor::ScheduleEarlyCheck() {
   ThreadTaskRunnerHandle::Get()->PostTask(
       FROM_HERE, Bind(&MemoryPressureMonitor::CheckMemoryPressure,
                       weak_ptr_factory_.GetWeakPtr()));
 }
 
 MemoryPressureListener::MemoryPressureLevel
-MemoryPressureMonitor::GetCurrentPressureLevel() const {
+MemoryPressureMonitor::GetCurrentPressureLevel() {
   return current_memory_pressure_level_;
 }
 
 // static
 MemoryPressureMonitor* MemoryPressureMonitor::Get() {
   return static_cast<MemoryPressureMonitor*>(
       base::MemoryPressureMonitor::Get());
 }
 
 void MemoryPressureMonitor::StartObserving() {
@@ skipping 131 matching lines @@
   return percentage;
 }
 
 void MemoryPressureMonitor::SetDispatchCallback(
     const DispatchCallback& callback) {
   dispatch_callback_ = callback;
 }
 
 }  // namespace chromeos
 }  // namespace base