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We follow the behavior of linux shell:
(1) Whenever there is a new query, the iterator is reset to the new
    query.
(2) Prev of the iterator moves till the first query and stick there.
(3) Next of the iterator can move beyond last query and get an empty
    query.


Bug: b:119504796
Change-Id: Iee1247359df78b8e2189fa86c3bd1b8b6ebfe612
Reviewed-on: https://chromium-review.googlesource.com/c/1334271
Commit-Queue: Yilei Li <yileili@google.com>
Reviewed-by: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609888}