mus: Fix closing windows created after the first window.
mus: Fix closing windows created after the first window.

Make sure the right connection maps the window id for the client.

BUG=none
R=sky@chromium.org

Review URL: https://codereview.chromium.org/1584053002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#369326}

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