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This cl fixes two issues:
1. When app_list is not active, we should not request focus to
assistant_page_view. Otherwise, the active window info will lose so that
we cannot get the assistant structure of the active window.

2. The DCHECK of |assistant_tree_| or |assistant_extra_| is not valid
because they could be nullptr if there is not such an active window.

Bug: b/139363094
Test: manual
Change-Id: I3b78688b15047925dc92f613fe9e46b198d864fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2017982
Commit-Queue: Tao Wu <wutao@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738284}