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This CL modify the media tray such that the bubble won't close when user
unpins media tray from shelf.

Bug: 1133488
Change-Id: I4ea32592c15b49312c8ab6a14e734069b39c9f5f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2439011Reviewed-by: Tim Song <tengs@chromium.org>
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Commit-Queue: Jazz Xu <jazzhsu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812244}