[heap] Reland "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..765533d0990322725252dd69fe7e24027670fdf0 100644
--- a/src/cancelable-task.h
+++ b/src/cancelable-task.h
@@ -27,6 +27,7 @@ class CancelableTaskManager {
 
   // Registers a new cancelable {task}. Returns the unique {id} of the task that
   // can be used to try to abort a task by calling {Abort}.
+  // Must not be called after CancelAndWait.
   uint32_t Register(Cancelable* task);
 
   // Try to abort running a task identified by {id}. The possible outcomes are:
@@ -40,7 +41,7 @@ class CancelableTaskManager {
   bool TryAbort(uint32_t id);
 
   // Cancels all remaining registered tasks and waits for tasks that are
-  // already running.
+  // already running. This disallows subsequent Register calls.
   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);