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When we try closing tabs, there might be 'unload handler' on them.
In this case, we should wait for the result. So in this case,
there's no closed web contents detached.

Bug: 842194
Change-Id: I78b0d9f8d42631b1309f4baa8dbbfa9503564426
Reviewed-on: https://chromium-review.googlesource.com/1248883Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: Sang Woo Ko <sangwoo108@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595375}