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Previously we used |std::shared_ptr| but a new pre-submit check blocks
that meaning we can not create new code using
|AssistantInteractionModelObserver| until this API is modified to use
|scoped_refptr|.

This is required for crrev.com/c/17564278

Change-Id: Id5963917160dee2761a35cb933e02616cc823122
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1766451
Commit-Queue: Jeroen Dhollander <jeroendh@google.com>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689981}