[heap] Cancel tasks before tearing down the heap.

src/cancelable-task.h

Index: src/cancelable-task.h
diff --git a/src/cancelable-task.h b/src/cancelable-task.h
index b1d62aad4bd595428d0b73da7497b67bca15a712..74afa6019cf9123d689aae39391854b6be19a1ca 100644
--- a/src/cancelable-task.h
+++ b/src/cancelable-task.h
@@ -40,7 +40,8 @@ class CancelableTaskManager {
   bool TryAbort(uint32_t id);
 
   // Cancels all remaining registered tasks and waits for tasks that are
-  // already running.
+  // already running. After this function is called all subsequent new tasks
+  // will be cancelled on creation.
   void CancelAndWait();
 
  private:
@@ -59,6 +60,8 @@ class CancelableTaskManager {
   base::ConditionVariable cancelable_tasks_barrier_;
   base::Mutex mutex_;
 
+  bool canceled_;
+
   friend class Cancelable;
 
   DISALLOW_COPY_AND_ASSIGN(CancelableTaskManager);