The mojo::Connector created by the ChannelAssociatedGroupController (ipc_channelbootstrap.cc) has its watcher posts tasks (in Watcher::CallOnHandleReady) instead of calling it directly.
Since all happens on the IO thread, it should call directly.

It seems that Watcher::task_runner_ is a sequence task runner for the IO thread but not the same as the one returned by base::ThreadTaskRunnerHandle::Get() on the IO thread.

Changing Watcher::task_runner_ to base::ThreadTaskRunnerHandle::Get() caused some child processes to still post, so it would probably be a good idea to tag Watcher instances created from ChannelAssociatedGroupController as should_not_post and assert if they pos so we catch these unnecessary posts.