Fix protocol for aborting and waiting for cancelable tasks.

src/cancelable-task.h

Index: src/cancelable-task.h
diff --git a/src/cancelable-task.h b/src/cancelable-task.h
index a6973d1e90e38907a439e7f6c6e6bed7299f70f6..a8387fcd950008e0b416e4cb09c6477a14334147 100644
--- a/src/cancelable-task.h
+++ b/src/cancelable-task.h
@@ -43,6 +43,10 @@ class CancelableTaskManager {
   void CancelAndWait();
 
  private:
+  // Only called by {Cancelable} destructor. The task is done with executing,
+  // but needs to be removed.
+  void RemoveFinishedTask(uint32_t id);
+
   // To mitigate the ABA problem, the api refers to tasks through an id.
   uint32_t task_id_counter_;
 
@@ -54,6 +58,8 @@ class CancelableTaskManager {
   base::ConditionVariable cancelable_tasks_barrier_;
   base::Mutex mutex_;
 
+  friend class Cancelable;
+
   DISALLOW_COPY_AND_ASSIGN(CancelableTaskManager);
 };