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Nodes are assigned to a slot only when nodes are host's children
and the slot is in the shadow tree.

BUG:869308

Change-Id: I10b9b58725d7550a7174a3f4a8e769a9f0c39996
Reviewed-on: https://chromium-review.googlesource.com/1176896
Commit-Queue: Kyoko Muto <kymuto@google.com>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584026}