[heap] Reland "Cancel tasks before tearing down the heap."

src/cancelable-task.cc

Index: src/cancelable-task.cc
diff --git a/src/cancelable-task.cc b/src/cancelable-task.cc
index defbb44775a8c1f64b8eedac62ea21b8376eacd1..5638e23fbf8a3674a51e2bbfa9f770f1396c8d56 100644
--- a/src/cancelable-task.cc
+++ b/src/cancelable-task.cc
@@ -26,13 +26,15 @@ Cancelable::~Cancelable() {
   }
 }
 
-CancelableTaskManager::CancelableTaskManager() : task_id_counter_(0) {}
+CancelableTaskManager::CancelableTaskManager()
+    : task_id_counter_(0), canceled_(false) {}
 
 uint32_t CancelableTaskManager::Register(Cancelable* task) {
   base::LockGuard<base::Mutex> guard(&mutex_);
   uint32_t id = ++task_id_counter_;
   // The loop below is just used when task_id_counter_ overflows.
   while (cancelable_tasks_.count(id) > 0) ++id;
+  CHECK(!canceled_);
   cancelable_tasks_[id] = task;
   return id;
 }
@@ -69,6 +71,7 @@ void CancelableTaskManager::CancelAndWait() {
   // of canceling we wait for the background tasks that have already been
   // started.
   base::LockGuard<base::Mutex> guard(&mutex_);
+  canceled_ = true;
 
   // Cancelable tasks could be running or could potentially register new
   // tasks, requiring a loop here.