Auto merge of #26036 - gterzian:add_ipc_router_shutdown, r=Manishearth

Add ipc router shutdown

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Explicitly shutting down IPC router as part of shutdown, for contexts see https://github.com/servo/servo/pull/25685#pullrequestreview-364604068

Note that the call to `shutdown` is idempotent, so it shouldn't matter whether the call in the constellation and the script-thread are actually on the same `ROUTER` in single-process mode...

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