Tests that prerender gets the UA override.
got: User-Agent: Lynx v0.1
got: sec-ch-ua-platform: "Lynx"
got: sec-ch-ua-mobile: ?1
{
    method : Preload.prerenderStatusUpdated
    params : {
        key : {
            action : Prerender
            loaderId : <string>
            url : http://127.0.0.1:8000/inspector-protocol/prerender/resources/echo-headers-and-broadcast.php
        }
        pipelineId : <string>
        status : Success
    }
    sessionId : <string>
}