Make memory pressure notifier configurable

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"
 #include "base/memory/memory_pressure_listener.h"
 
 namespace base {
 
 // TODO(chrisha): Make this a concrete class with per-OS implementations rather
 // than an abstract base class.
 
 // Declares the interface for a MemoryPressureMonitor. There are multiple
 // OS specific implementations of this class. An instance of the memory
 // pressure observer is created at the process level, tracks memory usage, and
 // pushes memory state change notifications to the static function
 // base::MemoryPressureListener::NotifyMemoryPressure. This is turn notifies
 // all MemoryPressureListener instances via a callback.
 class BASE_EXPORT MemoryPressureMonitor {
  public:
   using MemoryPressureLevel = base::MemoryPressureListener::MemoryPressureLevel;
+  using DispatchCallback = base::Callback<void(MemoryPressureLevel level)>;
 
   virtual ~MemoryPressureMonitor();
 
   // Return the singleton MemoryPressureMonitor.
   static MemoryPressureMonitor* Get();
 
   // Returns the currently observed memory pressure.
   virtual MemoryPressureLevel GetCurrentPressureLevel() const = 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_