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This CL makes slotting-manual mode support slot change event
which is already supported in auto mode.

BUG:869308

Change-Id: I106d1037036e5e34d552cf7ce17eacd8f453fc9c
Reviewed-on: https://chromium-review.googlesource.com/1179555Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Kyoko Muto <kymuto@google.com>
Cr-Commit-Position: refs/heads/master@{#585362}