Correctly handle the case when TimerTask is destroyed with being run.

src/heap/memory-reducer.cc

Index: src/heap/memory-reducer.cc
diff --git a/src/heap/memory-reducer.cc b/src/heap/memory-reducer.cc
index 2e0ccb9de6a07ebac1ea37141af7e25c7586f487..c89d1424e7c5e78e303ae64b7043669571bb4035 100644
--- a/src/heap/memory-reducer.cc
+++ b/src/heap/memory-reducer.cc
@@ -177,6 +177,13 @@ void MemoryReducer::ScheduleTimer(double delay_ms) {
 }
 
 
+void MemoryReducer::ClearTask(v8::Task* task) {
+  if (pending_task_ == task) {
+    pending_task_ = nullptr;
+  }
+}
+
+
 void MemoryReducer::TearDown() {
   if (pending_task_ != nullptr) {
     pending_task_->NotifyHeapTearDown();