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This CL converts |remote_cdm_| member variable to mojo::Remote
in MojoCdm class. As the change, this CL also converts related
things as well.

  - Convert mojo::StrongBindingSet to mojo::UniqueReceiverSet

TBR=xhwang@chromium.org

Bug: 955171
Change-Id: Id8f8e13fab7658ec6a94187febf9fcdad342340c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1841596Reviewed-by: Oksana Zhuravlova <oksamyt@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Gyuyoung Kim <gyuyoung@igalia.com>
Cr-Commit-Position: refs/heads/master@{#705902}