[iOS] Defer removing empty sections until editing is stopped.

The swipe to delete gesture for UITableView will throw an exception if
the deleted row's section is removed before the animation finishes.
This CL updates ReadingListTableViewController to defer removing empty
sections until self.tableView.editing is reset to NO at the end of the
swipe animation.

Bug: 877425, 877423
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I74e29b761fc8458e8bb9a6dbe643a73b00a8b9a4
Reviewed-on: https://chromium-review.googlesource.com/1198104
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589237}