This change ports all `EmbedderMsg` reply channels that don't use the
`ROUTER` to GenericChannel.
The remaining reply channels that use the router are blocked until
#38973 is merged.
This is a breaking change in the API between libservo and embedders.
Future work: A lot of the reply channels in this PR look like they
conceptually should be oneshot ipc channels. It might make sense to
provide a `OneshotGenericChannel` abstraction that encodes this.
Testing: No functional changes - covered by existing tests. None of the
channels changed here uses the Router
Part of #38912
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Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>
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