Don't run the second pass of the pending phantom callbacks if the heap has been torn down.

src/cancelable-task.cc

+// Copyright 2015 the V8 project 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 "src/cancelable-task.h"
+
+#include "src/base/platform/platform.h"
+#include "src/v8.h"
+
+namespace v8 {
+namespace internal {
+
+
+CancelableTask::CancelableTask(Isolate* isolate)
+    : isolate_(isolate), is_cancelled_(false) {
+  isolate->RegisterCancelableTask(this);
+}
+
+
+CancelableTask::~CancelableTask() {
+  if (!is_cancelled_) {
+    isolate_->RemoveCancelableTask(this);
+  }
+}
+
+
+}  // namespace internal
+}  // namespace v8