[Mac] Guard against null memory pressure dispatch source

base/memory/memory_pressure_monitor_mac.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_mac.h"
 
 #include <dlfcn.h>
 #include <stddef.h>
 #include <sys/sysctl.h>
 
@@ skipping 33 matching lines @@
           DISPATCH_SOURCE_TYPE_MEMORYPRESSURE,
           0,
           DISPATCH_MEMORYPRESSURE_WARN | DISPATCH_MEMORYPRESSURE_CRITICAL |
               DISPATCH_MEMORYPRESSURE_NORMAL,
           dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0))),
       dispatch_callback_(
           base::Bind(&MemoryPressureListener::NotifyMemoryPressure)),
       last_statistic_report_(CFAbsoluteTimeGetCurrent()),
       last_pressure_level_(MemoryPressureListener::MEMORY_PRESSURE_LEVEL_NONE),
       reporting_error_(0) {
-  dispatch_source_set_event_handler(memory_level_event_source_, ^{
-    OnMemoryPressureChanged(memory_level_event_source_.get(),
-                            dispatch_callback_);
-  });
-  dispatch_resume(memory_level_event_source_);
+  if (memory_level_event_source_.get() != nullptr) {
+    dispatch_source_set_event_handler(memory_level_event_source_, ^{
+      OnMemoryPressureChanged(memory_level_event_source_.get(),
+                              dispatch_callback_);
+    });
+    dispatch_resume(memory_level_event_source_);
+  }
 }
 
 MemoryPressureMonitor::~MemoryPressureMonitor() {
-  dispatch_source_cancel(memory_level_event_source_);
+  if (memory_level_event_source_.get() != nullptr)
+    dispatch_source_cancel(memory_level_event_source_);
 }
 
 MemoryPressureListener::MemoryPressureLevel
 MemoryPressureMonitor::GetCurrentPressureLevel() {
   int mac_memory_pressure;
   size_t length = sizeof(int);
   sysctlbyname("kern.memorystatus_vm_pressure_level", &mac_memory_pressure,
                &length, nullptr, 0);
   MemoryPressureListener::MemoryPressureLevel memory_pressure_level =
       MemoryPressureLevelForMacMemoryPressure(mac_memory_pressure);
@@ skipping 43 matching lines @@
     RecordMemoryPressure(last_pressure_level_, ticks_to_report);
 }
 
 void MemoryPressureMonitor::SetDispatchCallback(
     const DispatchCallback& callback) {
   dispatch_callback_ = callback;
 }
 
 }  // namespace mac
 }  // namespace base