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

This is follow-up for a5616e.

BUG=chromium:508584
LOG=NO

Review URL: https://codereview.chromium.org/1234513003

Cr-Commit-Position: refs/heads/master@{#29594}

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();

src/heap/memory-reducer.h

@@ -114,7 +114,7 @@ class MemoryReducer {
   // Posts a timer task that will call NotifyTimer after the given delay.
   void ScheduleTimer(double delay_ms);
   void TearDown();
-
+  void ClearTask(v8::Task* task);
   static const int kLongDelayMs;
   static const int kShortDelayMs;
   static const int kMaxNumberOfGCs;
@@ -126,7 +126,11 @@ class MemoryReducer {
    public:
     explicit TimerTask(MemoryReducer* memory_reducer)
         : memory_reducer_(memory_reducer), heap_is_torn_down_(false) {}
-    virtual ~TimerTask() {}
+    virtual ~TimerTask() {
+      if (!heap_is_torn_down_) {
+        memory_reducer_->ClearTask(this);
+      }
+    }
     void NotifyHeapTearDown() { heap_is_torn_down_ = true; }
 
    private: