Make memory pressure notifier configurable

base/memory/memory_pressure_monitor_mac.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_MAC_H_
 #define BASE_MEMORY_MEMORY_PRESSURE_MONITOR_MAC_H_
 
 #include <dispatch/dispatch.h>
 
 #include "base/base_export.h"
@@ skipping 10 matching lines @@
 // Declares the interface for the Mac MemoryPressureMonitor, which reports
 // memory pressure events and status.
 class BASE_EXPORT MemoryPressureMonitor : public base::MemoryPressureMonitor {
  public:
   MemoryPressureMonitor();
   ~MemoryPressureMonitor() override;
 
   // Returns the currently-observed memory pressure.
   MemoryPressureLevel GetCurrentPressureLevel() const override;
 
- private:
+  void SetObserver(MemoryPressureMonitorObserver* observer) override;
+
+  void Notify(MemoryPressureLevel level);
+
+private:
   friend TestMemoryPressureMonitor;
 
   static MemoryPressureLevel
       MemoryPressureLevelForMacMemoryPressure(int mac_memory_pressure);
-  static void NotifyMemoryPressureChanged(dispatch_source_s* event_source);
+  static void NotifyMemoryPressureChanged(dispatch_source_s* event_source,
+                                          MemoryPressureMonitor* monitor);
 
   ScopedDispatchObject<dispatch_source_t> memory_level_event_source_;
 
+  MemoryPressureMonitorObserver* observer_ = nullptr;
+
   DISALLOW_COPY_AND_ASSIGN(MemoryPressureMonitor);
 };
 
 }  // namespace mac
 }  // namespace base
 
 #endif  // BASE_MEMORY_MEMORY_PRESSURE_MONITOR_MAC_H_