Make memory pressure notifier configurable

base/memory/memory_pressure_monitor.cc

 // 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.
 
 #include "base/memory/memory_pressure_monitor.h"
 
 #include "base/logging.h"
 
 namespace base {
 namespace {
@@ skipping 10 matching lines @@
 MemoryPressureMonitor::~MemoryPressureMonitor() {
   DCHECK(g_monitor);
   g_monitor = nullptr;
 }
 
 // static
 MemoryPressureMonitor* MemoryPressureMonitor::Get() {
   return g_monitor;
 }
 
+void MemoryPressureMonitor::Notify(MemoryPressureLevel level) {
+  if (notifier_.is_null()) {
+    base::MemoryPressureListener::NotifyMemoryPressure(level);
+  } else {
+    notifier_.Run(level);
+  }
+}
+
 }  // namespace base