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Instead, use ArcSessionImpl's delegate.

BUG=b:142144019
TEST=ran the unit tests

Change-Id: I982d34ba82e93a31ea1e158a5d170e86eaf27a05
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1864326Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Commit-Queue: Yusuke Sato <yusukes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707416}