Make memory pressure notifier configurable

base/memory/memory_pressure_monitor_mac.h

Index: base/memory/memory_pressure_monitor_mac.h
diff --git a/base/memory/memory_pressure_monitor_mac.h b/base/memory/memory_pressure_monitor_mac.h
index b975b6ecec84ceb3dddef50fa1730fcf387ef2b4..2c5a591ad05b62143146e555ae1f8539053a24ac 100644
--- a/base/memory/memory_pressure_monitor_mac.h
+++ b/base/memory/memory_pressure_monitor_mac.h
@@ -28,15 +28,22 @@ class BASE_EXPORT MemoryPressureMonitor : public base::MemoryPressureMonitor {
   // 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);
 };