Tests that XMLHttpRequest Logging works when Enabled and doesn't show logs when Disabled for asynchronous XHRs.

XHR with logging enabled: 
VM:37 XHR finished loading: GET "http://127.0.0.1:8000/devtools/resources/xhr-exists.html".
makeXHR @ VM:37
makeXHRForJSONArguments @ VM:47
(anonymous) @ VM:1
VM:5 XHR loaded: 1
XHR with logging disabled: 
VM:5 XHR loaded: 2