Bug 1967805: use a modIterator for cancellable tasks;r=iain,arai

  AutoLockHelperThreadState lock;

  // Cancel all undispatched tasks.

  // We don't use an iterator here because we're releasing the lock,

  // and this way we don't have to worry about iterator invalidation.

  for (OffThreadPromiseTaskSet::Range r = cancellable().all(); !r.empty();

       r.popFront()) {

    OffThreadPromiseTask* task = r.front();

  for (auto iter = cancellable().modIter(); !iter.done(); iter.next()) {

    OffThreadPromiseTask* task = iter.get();

    MOZ_ASSERT(task->cancellable_);

    iter.remove();

    // Don't call DestroyUndispatchedTask() with lock held to avoid deadlock.

    if (task->cancellable_) {

    {

      AutoUnlockHelperThreadState unlock(lock);

      OffThreadPromiseTask::DestroyUndispatchedTask(task);

    }

  }

  cancellable().clear();

  MOZ_ASSERT(cancellable().empty());

  // When the shell is using the internal event loop, we must simulate our

  // requirement of the embedding that, before shutdown, all successfully-