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This CL converts MultiroomManagerPtr to new Mojo types.

It uses Remote and Receiver instead of MultiroomManagerPtr
and Binding.

Bug: 955171
Change-Id: I6615aff8b7ca9c58748bbd08ee08f38b57b4c202
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1804947Reviewed-by: Oksana Zhuravlova <oksamyt@chromium.org>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Julie Kim <jkim@igalia.com>
Cr-Commit-Position: refs/heads/master@{#697500}