[Mac] Guard against null memory pressure dispatch source

base/memory/memory_pressure_monitor_mac.cc

Index: base/memory/memory_pressure_monitor_mac.cc
diff --git a/base/memory/memory_pressure_monitor_mac.cc b/base/memory/memory_pressure_monitor_mac.cc
index 8548169c35b74ef64c78c957430efca0917452a4..391589021bd3cd6988fa10d53cf36535301e3b9e 100644
--- a/base/memory/memory_pressure_monitor_mac.cc
+++ b/base/memory/memory_pressure_monitor_mac.cc
@@ -51,15 +51,18 @@ MemoryPressureMonitor::MemoryPressureMonitor()
       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