In Bug 253342 we adopted `os_log_set_hook` so that we could relay logging information from the WebContent process to the Network process to add to the logging system.

The documentation for this method says:

 * os_log_set_hook returns the previously registered hook or NULL
 * when this function is first called. As a client, if you register a hook and the
 * return value is not NULL then it is your responsibility to chain the
 * previous hook into the new hook so that both are invoked.

Any previously registered hook is unlikely to be able to interact with `logd`, since we are sandboxed, but perhaps there are other side-effects we want to retain.

To comply with the API design, we should invoke any preregistered hooks before calling our own logic.

<rdar://112032774>