[heap] Activate memory reducer on external memory activity.

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index e1c8f5b841ec21ea01500976af5258c8f37eaac5..3ad6eb042cf93a432a4a957119d541170e626113 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -4516,10 +4516,12 @@ void Heap::CheckMemoryPressure() {
                               GarbageCollectionReason::kMemoryPressure);
     }
   }
-  MemoryReducer::Event event;
-  event.type = MemoryReducer::kPossibleGarbage;
-  event.time_ms = MonotonicallyIncreasingTimeInMs();
-  memory_reducer_->NotifyPossibleGarbage(event);
+  if (memory_reducer_) {
+    MemoryReducer::Event event;
+    event.type = MemoryReducer::kPossibleGarbage;
+    event.time_ms = MonotonicallyIncreasingTimeInMs();
+    memory_reducer_->NotifyPossibleGarbage(event);
+  }
 }
 
 void Heap::CollectGarbageOnMemoryPressure() {