[IndexedDB] Fixing early destruction of connection during forceclose

Patch is as small as possible for merging.

Bug: 842990
Change-Id: I9968ffee1bf3279e61e1ec13e4d541f713caf12f
Reviewed-on: https://chromium-review.googlesource.com/1062935
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559383}
IndexedDBTransaction::Operation IndexedDBTransaction::TaskQueue::pop() {
  DCHECK(!queue_.empty());
  Operation task = std::move(queue_.front());
  queue_.pop();
  return task;
}

IndexedDBTransaction::Operation IndexedDBTransaction::TaskQueue::pop() {
  DCHECK(!queue_.empty());
  Operation task = std::move(queue_.front());
  queue_.pop();
  return task;
}
